What is your opinion on paying with your personal credit card when a table pays you in cash?

I have a friend that I work with. He bartends and serves. Any time that somebody pays him in cash, he keeps the cash, and pays with his personal credit card. He acts like it is a really good way to build credit and to get hotel or flyer miles “points.” Idk if this is a really stupid idea or a brilliant idea. He always has tons of cash but deposits money in the bank pretty often, assuming that he’s always paying his credit card off. Does anybody else do this or know someone that does?

Editing to add: Our credit card system does charge a 3% fee or something close to that, so credit card payments do cost a little more than cash. That’s the part that I cannot understand. Why would he pay with his card if it costs more? Wouldn’t he be losing a little bit of his tip?

EDITING AGAIN TO UPDATE: I talked to him about it because I was worried about him getting in trouble. Our GM/part owner knows that he does this and approves of him doing this.