Would this be purely design choice?
I don't think this is a design matter. It is a matter of the flooring. You tile or whatever and it extends close to the wall - well doesn't matter a lot because vanity it over it. So there makes doing that side of floor easy instead of cutting the tile to fit exactly at vanity. And then that is the next point, what do you do if you get a new vanity? If it isn't the exact same size you would have issues. So easier to extend tile to wall (even if a little sloppy on that side) and this looks better, saves cuts/time tiling, makes putting the vanity in easier (shims), and can be changed out in a couple hours. Like shirlock said you just need a couple of screws through vanity frame to wall framing to keep it in place in the back and maybe one of the sides. I test after each screw and if there isn't a wiggle (I give a hard wiggle) then I stop putting in screws.
- Easier floor installation.
- Vanity to floor transition looks better - don't need skirt/trim.
- Easier to install vanity. I mean the vanity itself can take as little as 10 minutes.
- Much much easier to replace vanity or move vanity (think if you need to get to plumbing behind).
Most standard type box vanities are constructed with a back stile across the width of the unit at the top/back. We never connect them to the floor. We find the studs in the wall and screw the back stile to it. When doing this, you can correct the level both side to side and front to back. Strategic use of shims helps in the leveling process, but the screws lock the vanity in place.
A matter of design preference but also, in case of remodeling where you are not standing new wall studs, it may be difficult to suspend a floating vanity to existing studs, unless they are positioned to line up. I have a floating one because I totally gutted and rebuilt my bathroom and I positioned my wall studs specifically to be able to screw the vanity into them. If I had left the old studs, maybe it wouldn't have been possible.