Sqaures have overlap if you put them on a sphere, no?

There are some nice things with this. It contains cells at different levels (sizes). So you can use very small cells if you care about small areas, or large cells if you care more about larger areas. Those are given here https://h3geo.org/docs/core-library/restable/#average-area-i...

From a coordinate it is fast to get the cell at any level, so it's fast to group coordinates at whatever level you want.

If is less about comparing two coordinates, it is more about bering able to group large numbers of coordinates such that they go into non-overlapping groups, where everything in the same group is 'close' to each other.