Body scrubs are great for getting rid of dead skin cells and excess, stubborn dirt and grime that does not want to wash away with a casual wash.
Especially if you have sensitive skin, like up to 70% of the population, you are likely using a gentle cleanser, as you should, that may leave behind the harder to get rid of oils and other detritus.
Using a body scrub also prepares your skin to absorb whichever moisturizer you plan to apply after your bath or shower. And you must moisturize after your bath or shower!