First, to try to have a pool of repeat volunteers who we could rely on. In any other context, we would call these people "employees" but we ran on such a shoestring we couldn't pay everyone for all of their labor. Sucks, but such is the nature of the non-profit world outside of the glossy donor magazines. Most of these generous souls were recruited from friends and family or were people who'd received services from us in the past.
Second, is for advertising and to drum up support. People are far more likely to donate to a group they've volunteered with, even once, in the past as they feel a sense of ownership over the organization's outcome. Those "bring a bunch of six-figure salary folks to a group to pack boxes for a few hours?" They tended to result in some of the best donations we received for the whole year.