If you wonder why you need a wireless mouse, you dont need it.
Most people who use wireless mice are of two groups:
A) cables limit their usage pattern: they sit slightly farther from the computer, or they mouse with the left hand and the cpu is on the right, or they have serious cable tangle and they are simply trying to reduce.
B) they are gamers. Mousecord weight seriously affects the accuracy of mouse movement by adding inadvertant tugs and a directional pull to the cord. The weight of the cord itself, hanging off the end of the desk or otherwise, becomes an annoyance that the gamer finds him/herself constantly correcting for. Eventually, it becomes so annoying that he opts for a wireless mouse. For gamers its sort of like getting a serious plastic or glass mousepad. The friction from the mousepad messed them up, so they cut that issue out of the loop. Same deal with the cords.