When you sneeze or cough any mucus or other fluids are projected forward and into the inside of the mask. When you exhale, the air pressure in the mask increases and air is forced out the sides.

No, these masks are not bio-safety level 4, but they're probably better than nothing. At worst they prevent a portion of your sneeze and cough ejecta from landing on surfaces around you.

Coughs and sneezes are mucous suspended in the air. The face masks don’t stop “true” airborn infections, just ones where the infection vector is mucous.

It'll catch most of the large droplets.

Most of the large droplets fall to the ground instantly without a mask anyways, though.

It's the aerosolized smaller droplets that travel far and end up infecting others in your vicinity.