We are never told straight out why Party members have to volunteer for so many things, but we can infer the reason for this from what goes on in the book. Basically, the Party wants to make everyone volunteer for things so that the people can feel more committed to the Party's goals.
Totalitarian regimes often do things like this. They have quasi-voluntary organizations like the Hitler Youth or they organize "voluntary" activities where people do extra work (the Soviet Union encouraged people to join the Stakhanovite movement to do this). By having people engage in these extra activities, they hope to make the people feel like they are really a part of what the government is doing. If you are working that hard towards some goal, the hope is, you will come to believe in it even more.
We know that the Party wants the people in Winston's society to identify solely and completely with the Party and its goals. One way that they try to accomplish this is by requiring people to "volunteer."