why is martial arts useless for?

I think you are asking why martial arts is useless, and I have to ask what do you define as martial art? I define it as physical skill sets used to deal with violent conflict with another human being, taught to another individual(s) in a systemized manner.

By this definition, soldiers learn martial art of gunmanship, and I doubt you'll argue they are not effective in dealing with violence. MMA is more or less a sparring method that is becoming it's own systemized skillsets, and thus a martial art. People usually will not argue that MMA is pretty good at dealing with empty handed, one on one violence situations.

I think the issue is that your definition of martial arts is restricted to empty hand martial arts from the far east asia, like kung fu, TKD and karate. These styles often suffer from less than qualified instructors teaching the art and misrepresenting the style. This problem is farther compounded by the market they often draw in. They usually are dealing with teaching the kids or very casual adults who are interested in learning martial arts, but not getting hurt or having to be too dedicated(like getting too tired in training). Okay, that last bit I have a very specific person in mind, but essentially these classes draw in less than optimal type of person for molding out the martial arts master.

Which is often why you hear the phrase, "you should see the (insert style) in (native country)." It's because the people of the country has a better idea of what the style should be like culturally and will call out a person that is misrepresenting the art. Also the respect and dedication to the art in the native country is often better, leading to more dedicated students and farther leading to excellent martial artists.

OK. Please be more specific.

having an orgasm

What is martial arts used for?

or

Why is martial arts useless?

one gunshot is enough to silent your martial arts.