Well, the glasses is for two years, contacts is for one. It's because your eyes are always changing. 6 months, it could be a certain prescription, the next 6 months it could change enough to make a site difference. Plus your eye could develop all kinds of site inhibiting bacteria and diseases in a year or two, and the tests for those are sometimes part of the exam. If you purchase contacts without having a fitting it can mess with your eyes and really screw them up. They have to measure your circumference to be safe < worked as an optician.
TL;DR: Contacts without prescription can mess up your eyes, even if you don't need glasses. They have to measure your eye to give you the right ones, otherwise it could do a lot of damage < worked as an optician.