By leaving the grass clippings on the lawn you can recycle valuable nutrients into the lawn and reduce the frequency of fertilization.
Contrary to popular belief, grass clippings do not cause thatch to form in lawns, and decompose rather quickly since they are about 70 percent water. A properly mowed lawn should be two to three inches tall after mowing. Mow often so no more than an inch of grass is removed during each mowing.
Special recycling or mulching mowers chop grass clippings into smaller pieces so they decompose into the lawn more quickly. For optimum performance, the lawn should be mowed at least two times a week in the spring.
If large clumps of grass are forming when a lawn is mowed, it is because the lawn is not being mowed frequently enough or it is mowed while very wet from dew or watering. These clumps should be removed so as not to damage the lawn.
For more information, see the following Colorado State University Extension fact sheet(s).