Why do PhD students volunteer at conferences?
tl;dr No registration fees, networking and attending relevant sessions.

There has been some talk and background informationabout this already. See this, this, this and this for some background. With this background, IMHO, the primary reasons why PhD students should be volunteers for major conferences in their line at least once are:


First up, no registration fees. Yay ! Primary reason one.
Networking with other PhD student volunteers who are likely to be PhD students in similar departments working on similar stuff. For instance, most student volunteers at CHI 2013 are PhD students from HCI/CS/IS/ischools working on HCI stuff. Its great to meet with peers and bond with them. [Yay for student volunteer party !]
Possible networking with relevant faculty. This could be someone from the organizing committee or the program committee or someone else who you come in close contact with for the duration of the conference. There might be a certain someone who is a good researcher in your line and whom you might want to keep in touch with in the future. Getting in touch as a student volunteer in a conference might be one such strategy.
Picking and choosing conference sessions/tracks to attend. You could, with some dexterity be the student volunteer in charge of a track or a session or a panel which you are really really interested in watching. As a student volunteer pretty much, you stand and assist and you do get one of the best seats in the house, albeit by standing. :)