The point of my speed workouts is to increase my VO2 max, which is the maximum rate at which oxygen can be consumed and used by the body during a workout. From http://www.sport-fitness-advisor.com/ "As exercise intensity increases so does oxygen consumption. However, a point is reached where exercise intensity can continue to increase without the associated rise in oxygen consumption. This is the point of VO2 max." VO2 max can also be thought of as aerobic capacity, maximal oxygen uptake, and cardiorespiratory fitness.
Women usually have about a 30% lower VO2 max on average due to lower hemoglobin in the red blood cells (the oxygen carrier in the blood), and higher body fat ratios. Genetics also greatly affect VO2 max.
There is whole science behind VO2 max that I don't have my arms completely around. But basically, as you exercise at your maximum level for extended periods of time, you increase the body's ability to use oxygen more efficiently, thus increasing higher endurance and fitness level.
Speed work is one of most common ways to increase VO2 max. Typical speed workouts consist of shorter runs of 400M, 800M, or 1600M repeated several times at high speeds or intensities. Usually marathoners will do this type of workout once a week. For example, Bart Yasso created Yasso 800's where you run several 800M sprints at your target marathon pace. Meaning, if you want to run a 3:45:00 marathon, you run the 800M distance in 3:45. You then then rest for 3:45 and run it again, again, and again. He has experienced proven results with this method, and many runners report reaching their marathon goal time using these workouts.
I use speed work too. I know it's a proven scientific method that all professional athletes incorporate into their training regimens - but to this point, it seems like I have seen greater results by increasing speed and distance of my long tempo runs. This is basically a lower intensity way of developing cardiovascular capacity and thus, endurance.
The thing that I'm not sold on with speed work for marathon training is the constant starting and stopping. Typically during speed work, you run really fast for 400, 800 or 1600 meters, and then rest briefly, repeating several times. I think this type of training is mostly effective for shorter races like 5 and 10K's, because in a shorter race you can exert more energy for a shorter period of time with a short-term goal in mind.
However, a marathon is almost as much of a mental grind as it is physical. I don't see as much of an advantage in training your body for short, fast bursts when a marathon is a slower steady pace. The speed training definitely changes up the daily workouts and increases overall fitness - but mentally I struggle with them. The longer tempo runs at a faster pace than my marathon pace seem have a more direct correlation. But until professional marathoners stop using speed workouts, I'll keep doing them.