A lion's rough tongue is used for several purposes, including cleaning loose meat and gristle off of bones and also for grooming stubborn dirt and other muck out of theirs and their pride mates' fur. Lions are communal predators that require large amounts of nutritional intake, and they must keep themselves clean and healthy in order to maintain their grasslands camouflage and hunt effectively.
Lions spend large chunks of their days grooming themselves and each other. They do this by licking, much as house cats do, and their communal health benefits from these long periods spent in contact with one another and in the betterment of health and hygiene.
The reason a lion's tongue is rough, from a composition standpoint, is that it is covered in organic spines called papillae. These papillae are rougher than sandpaper and can scrape flesh off a human being with ease, but for the lion, they are the equivalent of a sturdy hairbrush or comb.
Because hunting is such an intensive experience and requires so much energy from the hunters, getting all the nutrition they can from the carcass is critical. The papillae on lions' tongues lets them make the most of their kills by scraping off the excess meat that would otherwise go to waste.