I thought that was to-do apps?

But the one you are making a comparison with is not really relevant: that's Apple-only it seems, and that, in itself, is enough incentive to look out for more.

Ultimately, market can decide which are needed and which aren't. As for investors like YC, they usually bet into multiple products — while YC might have a better hit rate than some other investors, there are likely still more failed investments than successful ones (by count). The important bit for YC is what the monetary equation is at the end.