She broke it so she could baguette properly....

From the grocery store's perspective, at least in the US, it keeps the checkout lines moving way faster when some kid who is trained to bag groceries does it, rather than waiting for the customer to figure out how to pack it.

All grocery stores where I live have this device consisting of swivelling plank separating the packing area into two. If someone is slow at bagging the clerk will simply move the plank and all groceries go into the second half where the second person can bag them. It gives the first person time to bag and the next customer again has their groceries place in the first half.
- We have these in my area as well, though these stores are mostly considered vendors for poor folk.
  Funny enough, the upscale stores have tons of self-checkouts and fewer cashiers. It's as if bagging one's own groceries with a cashier is for unfortunates, but doing both jobs yourself is in fashion.
You say this, but it never seems to be an issue here. Its a cultural thing i guess but people seem to manage without
Since most US people go to the market by car why do they need to pack? When I shop by car I just throw everything in the cart and then everything in the trunk, I only pack meat and some other moisty stuff
- We have a cultural practice here in the states where we display dominance to our neighbors by carrying all of our groceries in one trip.
  I’d look like a total removed juggling and dropping everything from the car to the front door.
  As it is right now, I deadlift my groceries and walk about 20 feet looking totally rad and dominant. I do this while loudly reminding my woman to “git” (which means something totally different from the git you’re used to) so the neighbors know that I’m a man’s man who doesn’t need any help from a female. That also shows her that I respect her weakness and don’t want her to get injured, which gets me laid later in the day. She rolls her eyes, but she knows how it is. This is why I have a pile of children by the way. All weak female children, but one day I will have a son and I will train him in the ways of his ancestors.
  Everyone always listens for their neighbor’s car too so they can step outside to giggle while they struggle and drop their groceries. Of course we also gotta help the ladies whose men have abandoned them to deadlift groceries by themselves.
  
  But seriously, why not put a bag or box in your car, put the groceries from the shopping cart in there and then carry that bag from the car to your home?
  
  See now you’re foolin’ worn tradition bud. I mean, maybe I can get my kids doing it. :p
  Not a bad idea though.
And yet somehow this isn't a problem at all in countries where we don't have people bagging our groceries. Checkouts very rarely have any downtime where we need to wait on people to finish bagging. Even with old people.
- Big difference between being walking distance from groceries and needing to drive there. Bagging a few days worth of groceries is a lot faster than 2 weeks worth
  
  A lot of people here go shopping by car for a whole week as well and we also have people living in areas where the next shopping isn't in walking distance. And it still works just fine. It's really easy because of course you don't have to stand in front of the checkout the whole time but can already bag stuff while the rest is being scanned. As long as you can bag your groceries as fast as they are being scanned it's really no big problem. And if you're a 90 year old granny who isn't as fast, they still can bag their groceries while the next costumer is being processed since the packaging area is usually divided into two sections.
  I'm really sorry, but the US didn't invent some special kind of super grocery shopping which no other country can understand. You just like the convenience of not bagging your stuff yourself. It's really not some top secret technique to make shopping 150% more efficient or something.

It creates jobs.

You're not the first one to say so, but a job that exist just to create jobs sounds like a bad excuse. Why would a store just create jobs?
- To create cheap (overworked) labor to make a customer's shopping experience better, in hopes of them coming back to spend more money.
  
  Checkout baggers are not overworked...
  
  As someone who works in a grocery store (but not on the front end), I like how you're assuming baggers only bag. I guess it depends on the company, but those poor bastards tend to have to bag, clear carts from the lot, sweep the store, clean the restrooms, clean spills around the store, among other tasks. All while receiving the lowest wage in the store.
  
  I'm not assuming that actually. None of those jobs are particularly hard. Check out baggers are not overworked.
  
  Neither is your smooth brain, apparently.

When you have a family of three and you're shopping for them by yourself and have to put them on the conveyor belt without help, it's a big time saver.

The people i see who have loaded carts, usually just pop everything back into the cart and repack either when they offload into the car or before they exit the store
- That's the way!
- That sounds like a great way to piss off other shoppers considering the lack of space for everyone to take the time for either of those activities in many supermarkets.
  
  I guess we must accept that things work differently in different parts of the world. It works fine here and no one gets either pissed off or is in the way.
  
  How much space is there inside the store for people to do that past the registers? Because here, there's room for about two carts and no table to do it on. Then in the parking lot, if the weather isn't terrible (it can get down to arctic temperatures and blizzarding here in the winter), you have to hope you leave enough room, if there is enough room to leave, for other cars to get by you.
  The only place I have ever been to in the U.S. where bagging your own groceries was feasible was Aldi.
  
  It's simply a different system. The markets are adjusted to it. There is enough room and sometimes even tables for this purpose. The markets are simply using different mechanisms to speed up the process so that they can have more customers per minute. Aldi is a big player here. The cashiers are trained and drilled to be super fast. There are no baggers, but enough room to pack your stuff yourself later. Recently they added a new kind of "double line". Now there is enough space for two trolleys right behind the cashier. Each line has their own card reader. The cashier has minimum waiting time for the next customer. Also almost zero time for customer interaction or any talk. Probably effective, but it really feels rushed out.
  
  The only way you could add space would be to take out and move all the checkout lanes.
  
  This doesn't pose a big problem for a company like Aldi. They are in the process of updating their older stores here so lots of things (big coolers, checkout lanes and baking stations included) get moved around.
  Or the whole building is designed to accommodate the different customer expectations in the first place.
  Globally operating companies can't just build one blueprint for every market, with few exceptions.
  See also (for a negative example): Why Walmart failed in Germany.
  
  You only need room for one cart behind the registers. They pack the same time the next customer gets scanned.
  And if people don't have enough space to get by you while you unload your cart into your trunk, then they should learn how to drive or buy a smaller car.

A lot of the time it exists to provide jobs essentially.