Some of the "pay-offs" of altruism, but within the more extreme circumstance of risking one's life to save another are address in this answer.
In my opinion, volunteer work fulfills a person's needs to be useful, it also places a person in a position of superiority, as they are bestowing their "gifts" upon a "lesser" person in need. There is evidence that charitable work releases chemicals akin to pleasure in the brain. [Dan Schulman]
I cite Ferguson, Eamonn; Farrell, Kathleen; Lawrence, Claire :
The benevolence hypothesis is supported, suggesting that blood donor
motivation is partly selfish.
Citing from the moral maze.
"Even in human societies we can still trace many behaviours seemingly
carried out for the benefit of the group back to benefits for the
individual," Stanford explains.
In the study Altruism Costs - the Cheap Signal from Amygdala, it was demonstrated that there a neurological changes within the brain when contemplating making donations.
neural processing of the cost-benefit difference. The presentation of
a charitable donation goal evoked an insula activity that predicted
the later decision to donate.../... Our findings imply that the
emotional system has an important role in real decision making as it
signals what kind of immediate cost and reward an outcome is
So in summary, I believe the pay-off is a feeling of well-being, that is in accord with supporting the individual's self-esteem and some evolutionary theories of our instincts as a species.
The Biology of Benevolence
Humans may be hardwired to cooperate. The choice to cooperate stimulates pleasure centers in the brain.
By Dan Schulman,
published on November 01, 2002 - last reviewed on May 02, 2006
Blood donation is an act of benevolence rather than altruism.
Ferguson, Eamonn; Farrell, Kathleen; Lawrence, Claire
Health Psychology, Vol 27(3), May 2008, 327-336
Altruism Costs - the Cheap Signal from Amygdala.
Gospic K, Sundberg M, Maeder J, Fransson P, Petrovic P, Isacsson G, Karlström A, Ingvar M.