Guys use these rings for different purposes:
Because they like the looks or want to draw attention to their penis.
They might like the feel of it and/or find it sexually stimulating.
In an SM game, to be tied up or to hang weights on.
To create firmer erections.
It’s also used by guys who have a hard time maintaining a hard on. When they use a cock ring blood is prevented from flowing out, prolonging their erection. It can be used in combination with a penis pump.