What's the point of different interval distance in the same workout or pyramid workouts ?

I think It depends, but generally speaking it helps your aerobic engine to perform better at higher speed, when you're tired.

When you do the short distance interval at higher speed after a longer interval, your body learns to be efficient at higher speed, even when you're not fresh.

In additin to what Starno said you can design workouts that don't beat yourself up as much or are tougher than the standard repeat the same distance over and over workout. In addition, it can keep you from getting bored, which can be important if you're training alone for months on end.

College coach for 20 years.... I've never understood the purpose of an interval workout that increases in distance (4x400m, 1x600m, 1x800m) and I've never had much luck with "brick" workouts (1x3200m, 4x400m, 1x3200m) for runners.

I've used a decreasing interval session (1x2000m, 1x1600m, 1x1200m, 1x800m, 1x400m) at times, but only sparingly.

I think the various length workouts are mostly used by coaches to keep their athletes mentally engaged with a new mental stimulus. I have not seen anything, from a physiological standpoint, that actually shows they accomplish much and if they athlete does not currently have an incredibly high fitness level you are probably doing more harm than good.

I think it teaches the body to run at varying paces and I personally like how I learn how different paces feel.

I'll add that it can teach the body to run at various distances too

I've been using pyramids every week since i started training. My reason is not to try out different paces. It's to know how to keep a steady pace. I run them all at 5k pace. The pyramid gets higher as the cycle progresses and then cuts back down with the taper and by the taper my 5k pace for the 400s would be faster but i now have to mentally focus not to go too fast but be comfortable knowing that i can sustain a higher pace. I'm a half marathoner so those paces are just another way to push up my race pace along with my tempo pace in training.

Pyramids, blocks, bricks, fartlek...

Just run baby