A life jacket has additional safety purposes to consider:
- A life jacket will help protect a person against hypothermia, an important threat when a rescue might be slow in coming. The jacket will help keep a person's head above water, so it stays dry, and it will help hold in body heat. Instead of exerting heat and energy in treading water, people wearing life jackets can pull their legs into their chests, which slows the escape of body heat.
- Most life jackets are brightly colored to aid in rescue efforts.
- Life jackets can absorb some of the impact of a fall and minimize injuries.
- Few people plan to fall overboard. When someone ends up in the water who didn't expect to be there, a life jacket can provide some valuable time in adjusting to the shock. *Most people thrash around when panicked; a life jacket helps the user right himself and keep the head above water.
The above information is quoted from an article on the "How Stuff Works" website.