Imagine the antenna pattern of any of our campus access points with integrated antennas to look like a "fat cone" where the top of the cone is at the AP, and the surface of the cone represents the direction with the peak gain.
By mounting the AP to the wall, you're basically putting that cone on its side.
While the coverage pattern looks quite different in both scenarios, there are pros and cons in terms of coverage for both. While designed/optimized for ceiling mount, the antenna patterns can be used for vertical mounting as well, but horizontal range may be reduced, and the coverage area will not be omni. You'll see significant bleeding into other floors as well.