Several reasons.
First, even though getting a career is that much further away, now is a good time to do the ground work as you have all the resources at your finger tips; Tutors, Careers staff and free internet access.
Secondly, forethought now will help you get the most out of your gap year. Having an idea of your eventual destination will enable you to identify activities which will contribute to your aim, for example gaining relevant work experience. If you are intending to use your year off to discover your direction in life, planning now should help choose promising avenues of inquiry.
There are almost as many reasons why people take a year out, as there are web sites devoted to the topic. The Gap Year website is a good place to begin as it gives a taste of the sorts of things different people do.
For more specialised resources see the Gap Year page in the Research Resources Section later.