The idea is that by subsidizing charitable donations, we as a society are getting greater positive social impact than we otherwise would. It's logical to ask, "How can we get the most positive social impact for our money?"
Whether we like it or not, government is an increasingly central player in social and environmental markets. After all, there is a clear public interest in the positive benefits impact investing can bring.
Although there are good reasons for a foundation to spend down its assets today, there are also legitimate reasons for it to continue its work in perpetuity.