Rhoa Mount, what's the point in increasing Spirit lvl. No info

What is the benefit of increasing the level of the Rhoa mount?
The tool tips have no info on what it improves at all for making it a higher level other than stat requirement.

Even Poedb says literally nothing.

This is a part of many problems with this game, trying to be so smart about build to the point there is no explanation for many, many things.

More hp for sure when i checked, assume all of its stats increase with gem level.