6" is a very common height off the floor. And it continues in 3" increments with 15" being the highest as plumbers tend to have issues after that.

Floating fixtures have two requirements: Visual and Cleaning. Visual: Being able to see under the vanity and toilet gives the room an "airy" effect that is very pleasing, and really changes the mood and presence of a small room that is otherwise hard to make a statement with. Cleaning: in small rooms there are sometimes weird spaces that are difficult to fit a floor cleaning appliance in. When you have wall mounted vanities and sink, the floor becomes much easier to clean. In my current bathroom I have a pedistal sink. My vacuum doesn't fit on the right side of it between the sink and the wall, and it doesn't fit on the left side between the sink and the shower surround. I can't vacuum on the left side of the toilet (too close to wall). This means the only effective way to clean this floor is on my hands and knees. If I had a floating sink and toilet, I could use any of my robots, vacuums, brooms, and mops, and they would cover 100% of the room.