I have volunteered at an animal shelter before. It is generally considered and altruistic activity where one works with no benefit to themselves.
I see, I think that morally it is good to participate in altruistic activities to help remember that there is more than just the self.
Yes, it is great and it promotes goodness and improves the human quality of life too.
 I agree, and on top of that it can be gainful for skill development.
That is true. Many volunteers are specifically trained in areas such as medicine, education or even emergency rescue. 
Many schools even have service learning programs to volunteer and earn credit simultaneously.
That is a great program!! Some others also just work on there spare time and some only work on an as-needed basis like when there is a natural disaster.