Life jackets are specifically designed to keep a person unable to swim on the surface for long periods of time. It has a collar that supports the head to provide protection in a case where the person is not conscious after falling in the water (for example an injury, contusion, hypothermia, breathing difficulties and so on). The jacket holds the person above water and turns them into a position where respiratory tracts are kept above water.
When wearing a life jacket, you can be certain that in the case of an unexpected fall into the water, there is plenty of time to choose the best way for a rescue. This is important because even though a person may be a good swimmer, unexpected variables (like cold water, strong waves or currents, being far from the shore, injury or so on) may significantly decrease chances of survival.
A child should only wear life vests designed especially for children, whether they can swim or not. In the interest of safety, children should wear life jackets even while walking in a harbor, on a wharf or on an anchored ship.