What does a PhD Committee do?
At least in the US, a Ph.D committee will have 4-5 members, and there are rules about the composition of the committee (there might need to be at least one person from an external institution, and at least one person from a different department, or variations thereof). 

The committee's formal job is to assess the Ph.D student's dissertation proposal, determine that the work being proposed is sufficient for a dissertation, and then evaluate the final dissertation defense and decide whether to grant the student a Ph.D or not. 

Informally, a Ph.D committee provides a set of resources/expertise for the student to tap into for advice, research directions and even contacts for future work (yes, there's life after a Ph.D :)). There's prior discussion on how to choose your committee.