Volunteering feels good.
A lawyer earning $500 per hour can use the money to hire 2 cleaners to pick up marine debris. If the lawyer volunteers to do the cleaning himself for an hour, he not only works slower than a professional cleaner but also offers his time for lesser value.
In a grand scheme of thing, the lawyer would have contributed more by using his money than his time.
Alas, we have a powerful urge to take part physically, even when our participations are not as aidful.