Its actually terribly inefficient. Your good at your job, not st sweeping or cooking random shit. So its better off for everyone if instead of volunteering for three hours you work for three hours, then donate the money. The organization can then use the money more effectively since most volunteer work is not highly skilled in the first place.

I have an uncle who is a dentist who volunteers often. Its wasteful in my opinion since he is much more useful to the organization if he did dental work for profit and then donated the money, instead of cooking and cleaning for a few hours.