It's for a number of reasons. The one was already mentioned- there's almost never a name that at least a few other people don't have, even for adult stars names, there may be a person with that as their legal name, so having xxx will help distinguish them from each other.
It's also to sort of warn people that the content they might post is x-rated. Like when you go to an adult website, there's usually a warning that you must click whether you're 18+ in order to enter. The xxx for twitter and other social media lets people know that there's going to be not safe for work related posts and photos.
And it's also for marketing. Potential customers and business partners will see the xxx and if they're interested in following them, it's easier to distinguish.