For the most part, where an NGO's money comes from has a lot to do with the size of the NGO. For example, massive organization like Green Peace need millions of dollars a year to operate, and those funds come from many sources: membership fees, interested businesses, philanthropic foundations, government grants, and so on. However, the smaller the NGO, generally the more reliant it is on a big collection of small donations, as opposed to a small number of huge donations.
A small NGO may operate on a few thousand dollars a year. This money is likely to come from friends and family of the people involved with the NGO. This is where grassroots volunteering can be very significant to the smaller NGOs like the ones promoted on this site: By promoting your experience, encouraging your friends and family to donate a few bucks here or there, you will likely help to fund a relevant portion of a small NGOs funding.
So, volunteering for a small NGO can be much more than helping to build a playground or gathering turtle eggs. It may just help to keep the program rolling in the years to come.