The idea of speed workouts, like "tempo" running workouts and interval workouts, is to increase your lactate threshold (LT). LT is the point at which lactate substances start to accumulate, which contribute to fatigue. Basically, LT is something you want to improve if you want to improve your distance running.
LT is related to VO2 max, which is where oxygen consumption plateaus. VO2 max is something I think of as a maximum effort point - it occurs at some maximum running speed and is also related to max heart rate. LT, on the other hand, occurs closer to "race pace." As pointed out at [http://www.sport-fitness-advisor.com/lactate-threshold.html], two runners with the same VO2 max speed will not necessarily perform the same in a distance race - the runner with the higher LT will perform better because the higher LT allows for a higher race pace.
Here is a link to a pretty good article that explains LT training:
In short, the above article suggests that tempo runs that involve running near LT (at about 95-105% LT pace) can improve the runner's LT. One example tempo workout involves 3-5 10-min intervals near LT, with 2-3min rest between intervals. The other example tempo workout is a 20-30min run near LT.