Sir Arthur Conan Doyle's pairing of the eccentric and brilliant detective with the somewhat stuffy physician home from military service is one of the most successful in literature. Whether in the original stories or in modern adaptations, the partnership works.
When asking whether or not Holmes needs Watson, there is no definitive answer. But there are opinions.
1. He needed help with his rent in the original meeting, so it is obviously the case that he needed him in that way.
2. Holmes also has a rather large ego, for which Watson serves as an outlet, allowing him to impress him with his powers of deduction - which spurs even more deductions.
3. Watson also serves as a foil for Holmes' theories, assisting him as he works through things he otherwise would not be able to do on his own.
4. Finally, Watson often would serve as a buffer between Holmes and danger, bringing a more logical approach to some of Holmes' more outlandish ideas.