Earlier quoted context omitted.
Part of the reason for floatation devices is a transitionary measure to get them more used to and practiced in "preswimming" while participating and not simply wading or pool side clinging. They are just often misused - you are supposed to be supervising them when they are in the pool period and they only need deep enough water to keep their feet off the ground.
Personal opinion, based off my own experience and teaching other children how to swim: Flotation devices have no part to play in teaching how to swim. Parents (or teachers) holding their children and teaching them how to float is step 1. Only after the child can handle themselves in water (float, know when to breathe) should they be playing with flotation devices.
And one skill that swimmers need, but cannot really get with an float-assist device, is putting their face in the water. I watched an adult friend learn to swim, and this was REALLY hard for him. Crazy enough - he was ex-Royal Navy submariner, so he had passed basic water survival - he could float on his back, just couldn't do anything beyond that.