Wall creation by inner line in Homestyler is a nice option, but it assumes that you will draw walls clockwise. But some people don't think that way (just look for some Sweet Home 3D tutorials on YouTube and you'll see that many users draw their walls counterclockwise!).
An other point is that Sweet Home 3D first idea was that users draw walls upon a blueprint they import, and not from scratch, and in that case, it's easy to draw walls from their middle.

Anyway, as said previously, if you want to use room inner dimensions, draw rooms with the Room creation tool, then activate the Wall creation tool and double-click in the room to create the walls around it.