Terrain considerations.
Most ground-based propulsion has some kind of surface that they cannot function well in. Wheels spin out in loose sand, legs can topple on builders, tracks lose traction in swamps and wetlands, and neither can move trough deep water.
Hover vehicles have none of these issues. Even if their altitude is limited to a few feet, they can cross any semi-solid surface without issues. They are also immune to most kinds of pressure-based landmines, and quad-copters can even cross chasms without problems.
Hovers crossing actual rivers and lakes depends on the way of hovering, but most types will allow that as well