Just Google [volunteering helps with] and you will get some reasons for which people volunteer.
But the most important reasons are these two:
- Instead of watching TV, you’ll feel better after you volunteer.
- Instead of working, you’ll also feel better when volunteering.
I think I’ve read it first on Derek Siver’s blog, but it was also a long thought of mine, that in order to best help an NGO you should do the work which is most impactfull (which is likely to be something closely related to your 9-to-5 job), and use the money you earn like this to help NGOs. This should have more impact than you giving flyers and organizing events.