Full-featured mice can, for instance, provide a customized experience on the go (with an onboard memory that stores various mouse settings), help you become a faster web surfer (with dedicated navigation buttons), or a more competitive gamer (with weight tuning, adjustable sensitivity, programmable buttons). Very tempting for the inner geek.
If you're a beginner though, you may feel overwhelmed by so many options. Keep in mind that the best mouse for you doesn't have to be the most feature-packed, but the one that suits your needs, first and foremost. If you just need a simple mouse that requires no configuration in order to fully use it, pick up a standard optical or laser mouse and save yourself some money.