You develop greater maturity and self-confidence by having a year away from school and if travelling, home. It allows you to 'break' from a school mind-set and become more aware of the wider world. It also allows you to develop practical skills such as budgeting money, coping on your own or surviving homesickness before you go to University.
If you get a paid job you can save money to help you pay for University expenses. It's also possible that whoever employed you might offer you vacation work or even work on graduation. Increasingly, all mainstream employers are looking for graduates who already have some general work experience not just a degree certificate.
Many vocational University courses such as architecture, nursing or law will be extremely interested if you spend your gap year on relevant work experience. Not only does this show your obvious interest in the subject but it often can provide invaluable practical skills.
It gives you something to talk about at University interviews, job interviews and graduate interviews - "Give me an example of when you..."
A gap year is an invaluable addition to your Curriculum Vitae or Personal Statement. It may be worthwhile mentioning your plans in your Personal Statement or in an interview prior to receiving university offers.