Eh.
Some believe it's a safety thing.
You can believe what you want but I can tell you from direct experience that coming out or off of a pedal is a safety issue.
Those that don't really push their bikes while shoulder to shoulder with 50 of their closest friends might not really see the point but having had to start a crit on flat pedals because I forgot my shoes was a completely eye-opening experience. After the first corner it was obvious I wasn't going to be able to produce enough power to stay in the game and I had WAY less control of the bike.
So yeah if you're just riding a coffee ride with a couple pensioners then no worries. If it's the Wednesday Night Worlds ride then...try it with some foot retention.