Working to improve public policy is often the best way to address the underlying problems facing the people many nonprofit agencies serve. It has been compared to "going upstream" to fix the railings on a bridge to prevent people from falling into a river, rather than only pulling them out after they've fallen in. In an ideal world we would do both.
Suppose there is a river with a strong current that washes people downstream if they fall into it. Upstream is an old bridge with broken railings. You could spend a lot of time and effort pulling everyone who falls into the river to shore. Or you could fix the railings so that fewer people fall into the river in the first place.