[tuukkah]

Hmm, you don't have to convert the geohash to a (base-32) string - int64 may be just as fine.

Perhaps they can take advantage of the better locality of Z index compared to flat index, and/or they want to store higher precision and do lookups with varying precisions (grid sizes)?

[Gen1us2k]

We use geohash because it's really fast and we can control precision. We don't need to store any information about grid and we don't need to convert coordinates to spherical view.