Who decides whats useful for not? Pretty much every company making anything thinks they are useful and they want to tell people how or why in order for people to buy them. They would need some way to tell that at scale, and ads are a way to do it.
I find that hard to believe. Companies do what is likely to make them profit, which sometimes involves doing things they surely know is bad for people. Cigarettes are still advertised in many countries; do you really think that the cigarette advertisers seriously think their product is useful?
If anything, I think you're more likely to get advertisements for the products that are relatively bad for people, since when your motive is to do good for people, you're less likely to oversell your product. It might be fine and well for this dynamic to resolve itself in a critical forum like HN, but when ads for random products are just thrown out to regular consumers, the rational thinking about how useful a product actually is will be minor.
So if computers are useful, shouldn't it be allowed to advertise them?
Overall, I do think that computers are useful, but if people are having to create an artificial demand for them, perhaps we're putting excessive amounts of labour into creating more computers instead of, for example, making them better. The vast majority of consumers aren't going to have a very good idea of what makes a computer better, so that's not usually the focus of computer advertisements.