I've always had a significant aerobic component in my running programs (about 35% anaerobic, 65% aerobic). These runs act as an "active recovery," enhancing blood flow and increasing capillary density (the enhanced microscopic network slows down the flow of blood past the cells, allowing more time for complete nutrient transfer).
The other poorly understood, but even more important benefit, is the increased ability of the body to generate more heat around the muscle motor neurons. Increased heat around the neurons lowers electrical resistance, allowing more current to pass. This permits more muscle fiber to act as fast twitch fiber!
The East Germans understood the role of additional heat when an extensive review of world record performances revealed how often the record setter was at the early stages of a cold and running a fever when the record was set. (Later into the cold, the adverse effects outweigh the benefits, of course.) This led the East Germans to experiment with de-natured viruses to generate a slight fever immediately prior to a world record attempt!
The warning here is that these "tempo" runs must not interfere in any way with the quality of the high-speed runs [read: the quality of your primary training]. This means that aerobic interval runs must not exceed 75% of your best possible speed. If your best time for the 200 meters is 21 seconds, then your interval 200 meter runs must be slower than 28 seconds! Additionally, your last interval must be as fast as your first. If you have any problem doing that, you're going too fast!