Because some guys try to use the "oops I didn't bring any condom" to try and rawdog it.
If it's a genuine mistake, then the gal can still have safe fun. Otherwise, if the guy is combative about the condom, it's a big red flag. Win/Win for her.
Most condoms are pretty one-size-fits-all. If you really want to make sure you have your bases covered you could always just cary multiple sizes of condoms.
I mean, you dont have to put the condom on a penis, you can put it on a toy to make clean up easier, and to prevent lubes from breaking the material down.