If the standard is zero transmission, then they are useless. But that isn't had has never been the claim. N95 is better than simple masks, but they are more expensive, initially were in short supply, and uncomfortable.
But this is all about statistics. Simple masks reduce the chance of catching it by some small amount (say 15%) and reduce the emission of virus particles by something like 50% (it all depends on the mask). This doesn't mean the odds are cut in half; it means the exponent of the spread of the virus is cut in half.
It's a simple and easy way to reduce chances of spreading it to another person. It diverts airflow from directly forward, changing the distance outward that aerosols from your mouth go. Since, as you point out, this virus is spread via aerosols, this impacts the chances of spreading it, by reducing the contagion radius around you.
In an ill fitted mask? Not well. The suspension of the virus in aerosol is precisely why masks have any efficacy in the first place, since the viruses themselves could easily permeate without it.
But this is all about statistics. Simple masks reduce the chance of catching it by some small amount (say 15%) and reduce the emission of virus particles by something like 50% (it all depends on the mask). This doesn't mean the odds are cut in half; it means the exponent of the spread of the virus is cut in half.