What's the clitoris useful for?

Nerve endings. The clitoris is the female equivalent of the head of the penis.
Sexual stimulation. The clitoris only has one function, it provides sexual stimulation. For some women, clitoral stimulation is required to have an orgasm. For many women the clitoris is so sensitive that direct stimulation is not pleasant at all, but causes discomfort. For those women, indirect stimulation, such as stimulation of the clitoral hood can provide the stimulation needed to achieve an orgasm.
Sexual pleasure. Make you feel good during sex, with foreplay and masturbation. Stimulation responsible in 80% of women who can experience orgasm.