I always assumed it was some sort of cuckolding thing or male chastity thing inflicted by dominant partners.
Wearing a male chastity cage can feel pleasant when it is the right size and fit. I feels as if the penis is being held tight or fondled, admittedly not by a hand, but still in a sexual manner. Walking around with one, when naked causes the cage to move around and pressure various parts of the penis in a subtle manner. When worn under clothing, the added confinement of clothing adds to the pressure and friction.
Wearing a cage, aside from the psychological satisfaction of the symbolism or relationship dynamic it carries and imparts, can be a purely physical stimulation of the male genitals, once again, depending on what kind of cage and the size and fit.
For instance there are cages that include a (short) urethral sound which must surely impart a special stimulation. There are other types that are designed to push the penis back up into the abdomen, supposedly to help shrink the size of the organ over time. That too must give a different kind of physical stimulation.
I'd say that ultimately at present, wearing a chastity cage on males, is a way to heighten sexual awareness and feelings, rather than to suppress a person's sexuality or horniness. Just like in medfet situations where examining genitals without going further into full out sexual play can be satisfying and arousing, so the same with wearing a chastity cage. It's just another form of restrained sexual play that is carried out long term.
btw: I've often wondered what the reaction would be if a male wore (a flesh colored) chastity cage at a nudist beach. I've seen enough guys wearing some sort of discrete penis jewellery at nude beaches, but no chastity cage - yet.