“Yes, people are sometimes limited in their ability to trade time for money (underemployed), so that it is better for them if they can directly donate that which they would usually trade for money. If the soup kitchen needed a lawyer, and the lawyer donated a large contiguous high-priority block of lawyering, then that sort of volunteering makes sense—that’s the same specialized capability the lawyer ordinarily trades for money. But “volunteering” just one hour of legal work, constantly delayed, spread across three weeks in casual minutes between other jobs? This is not the way something gets done when anyone actually cares about it, or to state it near-equivalently, when money is involved.” (Eliezer Yudkowsky, Rationality)

Relevant ideas here are Specialized volunteering.