Why? Most road runners, me included, do most or all of their training at a steady speed slower than race pace. Although this is good for endurance, it does not cause an adaptive response in the body because the body is already capable of this slower pace. Doing some training at a pace significantly faster than race pace makes abnormal demands on the body to take in, transport and use oxygen, to release energy. This ultimately produces an adaptive response, enabling us to perform better in races than we do during training. Because during interval training we push ourselves almost to fatigue, stop, recover and repeat, we improve our ability to recover quickly and tolerate fatigue.
You can tailor your speed work and recovery time depending on the distance you are training for. For 10K runners, do a rep that takes slightly longer than recovery time (about 30 seconds longer). If training for a marathon, do a longer rep at a slightly slower pace with with a shorter recovery. For middle distance, do shorter reps at a faster pace but with a longer recovery.
Ultimately, interval training should make us used to maintaining a fast pace even when we are tired, giving us that edge when it comes to races. So find a park, choose a lap and get going!