Masks can't stop individual virus molecules...I'm not even sure N95 masks do that. However, a virus is typically transmitted in a droplet which is much larger. Masks CAN stop these. However:
Masks do a better job keeping infected people from spreading droplets than an individual being infected because of them. In other words sick people wearing them help more than well people wearing them.
While masks can help, they really only help in specific circumstances. Namely being in close proximity to someone while talking to them face to face...or being near somebody who sneezes or coughs.
While a mask can stop droplets, the numbers I've typically seen are that they stop about 80%. I don't know that stopping only 80% is ultimately helpful...I'm assuming the other 20% can still make you sick.
While a virus can be spread directly through the air via droplets, I believe it is more common for it to be spread via touch from infected surfaces (i.e. someone sneezes in their hand, touches a door and then someone else comes along and touches the same door then proceeds to touch their eyes, nose or mouth). In other words, being careful about touching your face and washing your hands regularly probably helps a lot more than a mask.
So, sure, masks MIGHT help to a limited degree. And perhaps if the goal was to really "flatten the curve" as we were all originally told than maybe it makes sense to have everyone wear them for a short time. But it became absurd a long time ago...