"Why does my child need to use a float/noodle/discs/armbands?"
This is a question I'm often asked as a swimming teacher, especially by families whose babies learned to swim at a baby swim school without floating equipment. If your child has been taught to swim using adult led submersions and independent "swims", the notion of then swimming with a floatation device seems like a step backwards.
As I've explained in other posts, when children learn to swim without aids, the goal is to cover a given distance independently. Often, the child will hold his or her breath and perform a swim between two points, usually under the water, and it is a great skill to have - it can certainly be a life saving skill for those with a home pool. What it doesn't generally provide a child with is correct swimming technique. Children who have learned to swim independently between two fixed points often do so with a bent leg kick and a tucked body position, rather than the stretched out and streamlined body position and kick they'll need to make progress in swimming as a sport.
While breath control is a fantastic skill to have, in order to swim longer distances children need to find an efficient way to breathe. When you watch children used to swimming without a float, that point at which they put their heads up to take a breath often signals the end of their "swim" - lifting the head drops the feet, which pauses the progress across the pool.
Swimming without a float is exhausting - you can't correct a bent leg kick or incorrect body position when the child is using all of her energy to survive the allocated distance.
If we use a float we can correct a leg kick or tweak the streamlining and body position. Children need less breaks between swims because the float does some of the work for them - and that will lead to improved stamina. Any good teacher should provide plenty of opportunities to swim without the floats (just as all children should have opportunities to spend part of the lesson without goggles on if they use them), but work with floats will make up a big chunk of the session for a beginner or improver.
Floats can encourage children to put in less effort, but it doesn't take much to motivate them across.
Swimming with a float absolutely isn't a step back for any swimmer - consider it to be the next stage. If your child learned a survival swim with a baby swim school, they can build on that using floats. Michael Phelps trains with floats and pullbuoys - and if it's good enough for him, it's good enough for our children.