The fleece fabric doesn't hold on to water, but runs off quickly after a swim, giving you a feeling of being warm and almost dry. This is good for many immersion sports where you're in and out of the water all the time.
You can also use a fleece suit on its own for pool swim training or just for wet fun. It soaks up a lot of water making your resistance swim training really effective. When you come out of the pool, the water runs out quickly.
A fleece overall stays put under your nylon overall. This is important when you jump about a lot, in and out of the water. Nothing is more annoying when the clothes inside your overall sag and slip around.