For vertical mounting you can specify default mounting heights, so when placed in plan they are at the correct elevation on a vertical face.
They can interact with their host automatically, such as cutting recesses or penetrations without losing their host.
They are very easy to create and constrain in the family, there is no need to flex the thickness of the host element, as it doesn’t care about its thickness.
They are flexibly as you can apply them to any face, so if my basin needs to be hosted by a piece of casework it can!.
Why would you want to manually coordinate the alignment of a wall fixture with its wall if you don’t have to? Yes you can create an alignment constraint, but these types of constraints are manual and too many can really start bogging down the performance of a your file.
A face based family can be hosted to a face or a workplane, if you host a face based family to a level and delete the level, the family remains! A non-hosted family is essentially level based, if you delete the level the family is deleted. They are even more robust than an unhosted family with regards to hosts being deleted.
This is only the case for wall, floor, ceiling, roof based families.