Asking because I feel like sometimes if you check in as soon as possible they give you crappy seats, but if you wait for a bit the crappy seats are taken by other people so you get better ones.
No serious evidence, so asking here for other experiences
If you check in late for a flight that is usually busy, you have a higher chance of getting a premium seat for free (first rows or emergency exit rows with extra legroom).
I randomly got it once, felt like winning the lottery ha ha