Why is there a slab under my shower?

We are discussing remodeling our bath & putting in a whirlpool tub and seperate shower...I know this will get fairly expensive since all the plumbing is under the slab, but what I am trying to figure out, is why my shower is on a slab that is raised above the rest of the floor. Also, the drain is opposite of the faucet & knobs in the shower.

Probably because of the height of the drain.

Whomever installed that shower did not want to dig into the slab to route the drain line embedded in or below the slab. The only way to get a p-trap where it needed to be was to raise the shower up. It generally looks hokey, but does work. The cost to move it was probably the main consideration in why it is what it is.