Mulching is really good for plants because the grass takes all the oxygen and water away from the important plants. So putting mulch over the grass does two things. One - stops the grass from growing and two - the plants don't die they stay alive. Oh and one more thing, the mulch makes the garden nice and pretty.
Healthy Plants need water, sun and sometimes even some sheep poo that's sheep pellets because it is good and healthy for your plants.