When cutting on a regular basis, it is best to simply leave them alone. Grass clippings less than one inch in length will decompose quickly, providing nutrients to the soil. When grass is allowed to grow too long between mowings, the longer clippings tend to remain above the lawn, giving your yard an unsightly appearance. These clippings should either be bagged or removed.

According to research, returned lawn clippings have these benefits….
crabgrass and weed suppression, improved water infiltration into the soil, improved turf grass color, disease suppression, thatch reduction, and preserve soil moisture.