Well, first of all masturbation is due to sexual desires and many people, as well as children, use porn to become aroused. But there's a reason why porn is actually prohibited for minors. It can influence their sexual education in a bad way and it can also give children the desire to realize their unnormal fantasies with other persons, also with older persons who are likely pedophiles.
Apart from that many people, especially men, don't have a high after an orgasm as man, others do, but become sad or even a bit deoressed after masturbation. For them (us) it's better to avoid masturbation because that way they can actually live an overall happier life. You can sort of compare it to alcohol or drugs. You have a good time when you orgasm, but after it's gone it's nit nice anymore and after some time you also have the desire to masturbate again which becomes stronger day by day.
Avoiding masturbation gives me a way happier life.
Also, it's just a nice feeling of safety that you have when you are locked. Of course the belt does have discomfortable aspects, but the fact that your genitals are covered closely by the warm "hand" of the belt just feels really good. Might also have to do whith the fact that some people / men like to slide their hand under their pants without masturbating.