A person with spare money to donate can either make the donation immediately, or invest the money and donate a larger sum at a later time.
There are good reasons for investing and donating later. First, giving later means that the donor will earn interest on their money, and so will have more to give away. Second, waiting allows them time to gain more information about where to give, and potentially increase the effectiveness of the donation. Thus, waiting allows donors to give more, and to give better.
However, there are also good reasons for giving now. First, giving now creates a habit of giving, and removes the temptation to spend savings which were originally intended to be donations. Second, giving now allows a donor to build capacity in areas that they may want to fund later, and to incentivize others to work in those areas. Third, there are often also tax benefits to giving in the year the money is earned. Fourth, a donor may discount the future, and the rate of time discounting may more than offset the rate of return on investing.
Some of these reasons have general applicability, whereas others apply to specific focus areas. In particular, a key question is whether interventions in the selected area are likely to become more, or less, effective in the future. The problems addressed by the most effective interventions might get solved, reducing future effectiveness. On the other hand, innovation might make future interventions more effective.
If donors are especially convinced of the value of donating now, they could potentially borrow money, give the money to charity, and pay the interest off in the future. This can, however, be a risky thing to do and will not be appropriate for many financial situations. A similar mechanism is to commit to giving to a charity for a number of years, thereby allowing the charity to count on future donations without any instant cost to oneself.
Christiano, Paul. 2013. The best reason to give later.
Cotton-Barratt, Owen and Ben Todd. 2015. Give now or later.
A discussion of a framework for considering this question developed by 80,000 Hours and the Global Priorities Project.
Karnofsky, Holden. 2015. Good Ventures and giving now vs later.
A discussion of this question from the perspective of a large institutional investor.
Mannino, Adriano & Lukas Gloor. 2014. The case for donating now.
A discussion of the haste consideration.
Wage, Matt. 2012. The haste consideration.
Wise, Julia. 2013. Giving now vs. later: a summary.