Fabric over the face does offer a basic filter. But yes, it doesn't do very much, if anything. At the most it slightly reduces the distance that particles will travel in the air when you exhale or sneeze. But it won't filter out virus particles. You need a high rated mask n95 or n98 to really have an effect on that.

Also, most masks that have a high filter rating are only effective at that rating for about half an hour or so.

As the material becomes saturated with moisture and particles from your mouth it stops working properly.

If you're reusing a surgical mask for days, it is no longer effectively filtering virus particles.