You don't have to wait for Izzy to do its thing, which may take days to get an update to you. This will grab from the source directly without a middleman, checking the source however many times per day that you set it to. I guess you could consider Obtainium to be a middleman, but that's better than relying of Izzy and F-Droid to be your middlemen.
Also, not everything is going to be available on an F-Droid repo. Just off the top of my head, all these new Lemmy clients popping up. Because they are so new, I doubt you'll find them on Izzy yet.