Matchers may not know what needs their help the most. If you were a wealthy person and wanted to donate, but weren't sure what a good cause was, you could match what other people donate. Person A and B could come to you, and ask you to match their donations to charity C and D. Without knowing anything more about the people or the charities, matching acts as a proxy for which one is more important. The person who spends more is likely a better use of your money.

From a company point of view, it's advantageous to match. If two employees want the company to donate to their pet charity, then matching is a way to not accidentally play favorites.