I play right-handed. If I sit, bass rest on the left thigh. For me, this just positions the bass in a position that is similiar to my standing position and it keeps the neck raised. Also, for me, it just allows for better access to the upper register on the instrument. If I rest the bass on my left thigh it is a bigger transition when I stand and the neck is in a more horizontal position. Different strokes for different folks . . .
Same as above.