They offer no protection to the user, only to other people if the user is infected.
Exactly. They can potentially help to control the spread of the virus and protect the vulnerable. The evidence for their effectiveness is not really firmly established yet and they may be of little use but it's a point of principle and a question of attitude. When people are not bothering to wear them despite the fact that it's mandatory, what does that tell you? When people are wearing them hanging round their chins instead of covering their nose and mouth, what does that tell you? When people don't maintain distance from others where possible, what does that tell you?
Infection rates are always a function of testing
most testing reveals non-medical infection
A very good indicator is the percentage of positive results being obtained. As you increase testing to include those who don't show any symptoms, the positive percentage of test results drops, so you get a more realistic picture of the rate of infection.
Btw the latest figures for Poland show 440 new cases and 15 deaths.