Should a bathtub be installed on a concrete slab or the subfloor that will be installed over the slab?
In most cases if you place the bath tub directly onto an existing concrete floor you will then have to remove the concrete in the area around the bathtub drain to permit room for the drain basket, drain pipe and P-trap. Sometimes you get lucky and the bathtub drain lines up perfectly with a drain line that comes up just through the concrete floor .. and .. the necessary P-trap is already in place under the concrete surface. 

If this is not the situation that you have then it may be necessary to have the tub on the elevated floor. This can leave just enough room to route a drain pipe from the tub drain over to where there is access to the necessary further parts of the drain line.