There are many reasons for using grass clippings as mulch in your garden.      

Use of grass clippings - or straw - is beneficial for garden in many ways, but there is one condition to make it work best: the thickness of the layer must be at least 3-4 inches, the more - the better.

Reasons:   

a) significant reduction of weeds - they just die under the thick layer of mulch, and those grown thru are very weak and are pulled easily by hand.

b) significant water saving - soil does not dry out as quickly under the layer of mulch, keeping roots of your plants moist even in hot days.   

c) grass clippings provide constant organic feed for your plants as they are rotten slowly from the bottom up, and providing shelter and food for "good" microflora in the soil.

d) grass clippings keep your harvest clean: strawberry fruits, tomatoes, cucumbers - without touching the bare ground they are nice and clean.   

e) when used as a ground cover between rows - it becomes a carpet you can step on even in muddy rainy days to pickup greens for salad and have your feet clean   

f) it improves your garden year after year returning valuable nutients back to the soil without adding any chemicals, that essentially increases your harvest and keeps your food save. 

g) it is FREE and often is thrown away, so you can pickup tons of it from curbside on garbage collection day nicely packed in paper bags by your neighbors.