If you cover your mouth and nose with anything, it may help if someone coughs or sneezes near you (but wash it when you get home). I think that those of us who are re-using masks over and over again might as well just use a scarf, quite honestly.
It depends on the seal, see below:
It's just the fact the mask blocks it on the way out supposedly, if you cough, but not on the way in if someone coughs on you cos the tiny particle goes through the mask.
When you breathe in or out, if there is a good seal between the mask and your face, the air flow will be forced to go through the filter element. My understanding is that this will slow down the air sufficiently for the particles/droplets to become trapped.
Certainly without a good quality seal, you will inhale and exhale some air through gaps in the seal region, rather than through the filter element. High density (compared to air) particles/droplets which are ~1 micron (typically) and are already suspended in the air are excellent at following the air flow. There is no debate about this, it is well known that such particles/droplets, once suspended, follow air motion extremely well. So if there are any gaps in the seal, there is a risk you can inhale them, and also a risk you can exhale back into the atmosphere.
If you want to wear the mask as a means of preventing others from becoming infected, this is risky, at least for some masks. As the filter blocks particles/droplets, it can become more difficult to breathe through because the filter "clogs up". This idea that you can "clean" the disposable masks (and therefore allow you to reuse them) may not be true. I don't know how the masks are designed, but I believe the filter element of some of the masks may just be layers of fabric, so you may be able to clean the outer layer well enough but not the inside layers.
This idea that you "don't touch your face" makes sense in theory but in practice, you will still scratch an itch, rub your eyes (for half masks) and risk removing the mask incorrectly and therefore touching your face. You should not be using a mask for the sole intention to avoid touching your face. If this is someones motive, they are much better off just wearing latex/nitrile gloves.