I just think they sleep better and longer while flat. Sleeping flat promotes better sleep habits, for a lifetime. My sole purpose in life after each of my babies was born, was faciliting their good sleep habits, with the goal of getting them to sleep through the night ASAP! All of mine reliably slept through by 3 months old...one of them by 10 days old. And my babies all initially had their days/nights mixed up, just like most babies, and 2 of them had colic. I swear by flat sleeping. And a few other tricks.
And, yes, it keeps them from flopping to the side or forward.