Someone correct me if I'm wrong, but after having a chat with one of my mates recently, I think it's because some people with big asses have to do this because their cheeks make a seal around the seat if they remain sat down. My mate was talkin bout how he has to spread his cheeks manually when shitting and it was a bizarre bit of information to digest

Funnily enough, I'm typing this while having a shit, just fyi