Some people think the best reason to give now is "giving to learn" — that is, giving to learn more about which interventions are most cost effective. E.g. both GiveWell and Paul Christiano have written about this recently.

GiveWell lists "giving to learn" as the #1 reason they recommend particular near-term grants.

Christiano: "I think the most important impact of giving now is probably that it accelerates the process of learning... A relatively small set of activities seems to be responsible for most learning that is occurring (for example, much of GiveWell’s work, some work within the Centre for Effective Altruism, some strategy work within MIRI, hopefully parts of this blog, and a great number of other activities that can’t be so easily sliced up)." [I would add some strategy work at FHI.]