[aalam]

It's not always social status, especially for people who volunteer but don't tell anyone. It can often be ideology and principles, from people who want to pay a favour forward (e.g. if they received help from the past).

I do agree there are other intangibles, though (sense of community with fellow volunteers, sense of freedom by doing something exceptional, work experience). These intangibles are good things (mutual benefit).

Some people also benefit without expectation of reward. There could be an argument of self-interest (e.g. to feel good or alleviate some past wrong), but in practice, it makes sense to just thank these people for their actions without worrying about their motivations (which they might not even definitively know).