That's like asking what's the point of keeping your cash hidden in your wallet in your pocket instead of wearing it in a clear bag around your neck... if people know you have a large amount of cash, or are worth a bit, you become a target and potentially put yourself at risk for displaying your net worth. You don't want the world knowing how much you have on your person or in your bank account, so I'm sure you don't publish your bank account balance publicly.
Because of the nature of cryptocurrency and their open ledger process, mixers are a great way to provide privacy from potential hackers and scammers from making you a target by being able to see your balance or how much crypto you move. There's nothing illegal about wanting privacy.
I think a lot of people fail to realize that good ol' cash is still used on a daily basis for more illegal purposes than Bitcoin is.
Mixers don't promote illegal activities. They promote privacy. Nothing wrong with that!