You often want to maintain reach when increasing rise. Geometry dictates if you start with a 120mm 90degree stem, if you were to replace it with a 130degree riser stem, you'd need 135mm extension. (at the very least -- as your hands come up they can reach further forward, so you often want to both increase rise and reach, say if you are fitting someone 6'4" to the largest bike your shop sells and still coming up short.)

Stem manufacturers seem to have no grasp of this geometry though.

I just bought one of these: +40 degree rise, 180mm extension