There are multiple kind of problems. One of these is that they use your power, and waste your network bandwidth, to try to force to display them and to track y

There are multiple kind of problems. One of these is that they use your power, and waste your network bandwidth, to try to force to display them and to track you and other stuff. Their use of your power in this way is like theft; they are stealing your power for their own use (even though such theft is not being done directly). Although you could (at least sometimes) block them, they often make it unnecessarily inefficient and difficult. Other problems include both A and B above, but these are separate problems. There are additional problems as well, such as deceptive advertising, and others. (TV shows that display advertising within the show is also a problem, but again that is a separate problem than any of the above.)