This is a genuine but possibly stupid question. In high school, my coach had us do a good number of interval workouts where the distance of each repeat changed in a "ladder" or "pyramid" style. 800/600/400/400 at mile pace, for example. In college, my team never did any "asymmetric" workout--it was always x number of of Y meter repeats at Z pace (save for quick 200s after a workout near the end of the season). Our women's team, however, did a LOT of "pyramid" style interval workouts like 4x200m,2x400m, 800m, and back down again, often varying the speed of the repeats too. I've been going over some of Renato Canova's workouts that he has his elites use, and he also does a lot of asymmetric stuff (3k, 2k, 2x6x400, for example). In the very small amount of information I've garnered on what Salazar has his runners do, I've seen a lot of varied interval type workouts. One with Galen Rupp on flotrack has him doing three sets of 600/400/300/200, with the shorter repeats faster.
My question is this: What's the logic behind these types of workouts? Is it of mental or physical benefit to do a long bout at race pace early on in a workout, then do shorter bouts later? Or vice versa? The main argument I've read for "normal" interval workouts like I did in college is that the difficulty ramps up in a predictable fashion--this isn't the case when you switch up the distance or the speed. Also, is changing the speed during a workout beneficial? Most these workouts get faster at the end, is this to practice speeding up at the end of a race? When should you use basic intervals (x by y at z) and when should you use a more complex structure?