Marathon-pace progression runs are an effective means to increase the challenge level and race-specificity of long endurance runs. Many runners make the mistake of doing all of their Saturday or Sunday long runs at a moderate pace, but once you have used these runs to develop sufficient raw endurance, they don't provide any further benefit unless you pick up the pace.
You don't have to be training for a marathon to benefit from marathon-pace progression runs. They provide excellent aerobic support for any race distance, although you will want to use them differently depending on your specific race distance. If you're training for a 5K or 10K, marathon-pace progression runs should be emphasized relatively early in the training process and then phased out in favor of long runs that include even faster running.
If you're training for a half-marathon or marathon, they should be emphasized later in the training process, and they should be longer. A good peak-level marathon-pace progression run, appropriate for three to four weeks before a marathon, is two miles at a moderate pace followed by 14 miles at marathon pace.