The whole point about meditation is about self-reflection. Yes, you may argue that comparing meditation with volunteering, the former would seem like a complete waste of time with no benefit to society, but its only because you're looking at it in one-dimension.

If the OP is able to turn his life around, become calmer, more understanding and more compassionate, his actions and interactions in the physical world would have an impact to all those around him and create a multiplier effect that is much more effective in the long run than just pure volunteering.