PhD supervisors usually get paid to supervise. They also have the prestige of having lots of students under their belt.
It's common to also have co-supervisor (I think it's mandatory in many fields, in Australia at least). What do these people get in return for their services? Put another way: if I'm asking someone to be my PhD supervisor, expecting them to help me out a whole lot for the next 3-4 years, what do I have to offer them?
Co-supervisors play different roles, which may vary from individual to individual. Sometimes they are there as a backup, either in case things go wrong with the main supervisor, or if the student needs someone else to talk to. They can read the papers (and eventual thesis) of the student and may even participate in the research, adding a different perspective. In some cases, they can be more active than the supervisor (this is often the case if the co-supervisor is a post doc). Often they will be a part of the committee assessing the thesis in the end.
What does the co-supervisor get out of it? Well, a tick on his/her CV. Publications. Experience dealing with students. Contacts. Ideas. And work. In any case, it is part of what an academic is paid to do.
The real question should be: what do I, as a PhD student, get out of having a co-supervisor?