It's not about the large donor making a fixed sum donation - they're using the large donor to encourage even more donations with the matching, getting them more

It's not about the large donor making a fixed sum donation - they're using the large donor to encourage even more donations with the matching, getting them more money than they otherwise would have in a single lump sum. It's quite possible that the large donor has said I'll match up to $X even if it takes a while to reach that goal - effectively the same as a lump sum. Especially if they're able to use this to establish recurring donations or reach out to more potential donors when they're fundraising, it can really pay off down the line. Other people have said maybe this is a way for the large donor to optimize their donations, but personally I doubt it, there would be limited benefit at best to that, it'd only find the most popular charities, not the most worthy and those numbers are trivial to look up. If I had to guess, this is probably something the charity suggested when they were offered a large donation because they can make more from it in the end.