Well as so often the justification these days are getting hared to come up with.
There was a a nice time where the answer was simple and clear.
You get rid of lactic acid .
As the majority of coaches and people today accept the fact, that there is no such thing like to get rid of lactic acid, as even there is no need to get rid of lactate.
If we accept the idea, that lactate is a great and valuable energy source,
than why would you try to get rid of it in between workouts and or in between intervals.
Why would you " burn " of a great energy source, when it fact it would speed up re-loading of energy storage , when we would not burn it of.
So a " cool down" for sure will drop lactate as we need energy to cool down and lactate is nice enough to support the energy demand. And we though that is great but today we could argue that this is NOT that smart , as we could use it for the next workout, if we would store it.
So there is an interesting old idea, that when lactate disappears faster , than that is a good thing .
A good thing for what ?
Here an very fun study which will add some fuel to this above questions.
CHOI, D., K. J. COLE, B. H. GOODPASTER, W. J. FINK, and D. L. COSTILL.
Effect of passive and active recovery on the resynthesis of muscle glycogen.
Med. Sci. Sports Exerc., Vol. 26, No. 8, pp. 992-996, 1994.
The purpose of this investigation was to determine the effect of passive and active recovery on the resynthesis of muscle glycogen after high-intensity cycle crgometer exercise in untrained subjects. In a cross-over design, six college-aged males performed three, 1-min exercise bouts at approximately 130% VO2max with a 4-min rest period between each work bout. The exercise protocol for each trial was identical, while the recovery following exercise was either active (30 min at 40-50% VO2max, 30-min seated rest) or passive (60-min seated rest). Initial muscle glycogen values averaged 144.2 +/- 3.8 mmol-kg-1 w.w. for the active trial and 158.7 +/- 8.0 mmol-kg1 w.w. for the passive trial.
Corresponding immediate postexercise glycogen contents were 97.7 +/- 5.4 and 106.8 +/- 4.7 mmol-kg-1 w.w., respectively. These differences between treatments were not significant. However, mean muscle glycogen after 60 min of passive recovery increased 15.0 +/- 4.9 mmol-kg-1 w.w., whereas it decreased 6.3 +/- 3.7 mmol-kg-1 w.w., following the 60 min active recovery protocol (P < 0.05). Also, the decrease in blood lactate concentration during active recovery was greater than during passive recovery and significantly different at 10 and 30 min of the recovery period (P < 0.05). These data suggest that the use of passive recovery following intense exercise results in a greater amount of muscle glycogen resynthesis than active recovery over the same duration.