Earlier quoted context omitted.
I'm not an expert, but I believe they stop the droplets, which are vectors for viruses.
It has been proved that most of those masks sold simply aren't closely sewed enough to stop the droplets containing the disease. So in general the mask don't help. Secondly the mask only help avoiding spreading the virus from yourself to other and it doesn't protect from receiving the virus
The specific question that is relevant is, given droplet exposure containing COVID, are there any differences in infection rates for people wearing a mask (use different levels of mask protection for comprehensiveness, from bandana to N95) versus without a mask?
A mask can be only 10-20% effective but still show a clinically relevant result. If you think about the effectiveness of washing your hands to prevent getting sick, it is probably a low reduction but it is still highly recommended.
People do not argue against hand washing just because the virus can still get into your system through other means. They support hand washing despite its imperfections.