It's easier for kids to start pedaling when the other foot is on the stool, so they're balanced while starting.

Once they got it they will find natural stools in the park and use them, until they don't need them at all.

There is some difficulty in starting pedaling: you need to simultaneously give a boost to the bicycle, balance yourself on a non-balanced bicycle, and leaving the breaks. The stool remove one hard task - the balance.

BTW till this day I still search for natural stools when I stop at lights, it's just more comfortable to stay this way, more balanced.