if you think the Future of Humanity Institute (FHI) currently has a funding gap of $100,000, you might make a single-player analysis that donations up to this limit will be about as cost-effective as FHI on average. But in fact, if you don’t donate, then additional donors will probably come in to fill the gap, at least over time, making your donations less effective than they first seem.
On the other hand, if you think other donors will donate anyway, then you might think your donations will have zero impact. But that’s also not true. If you fill the funding gap, then it frees up other donors to take different opportunities, while saving the charity time.