alright, well my reasearch in working out and theory and all that good stuff has lead me to two theories:
1) The more you do something, the better you get at it.
2) All abilites have carryover to other abilities.
So basically it works like this ;): say you have a workout of 4×200 at 30 seconds each and 60 second rest (random example), then your body will adapt itself to that workout. so if all you do is this one workout, you will get good at doing 4×200 at 30 seconds with 60 seconds rest. you will also be able to run 200s faster and slower because it has carryover. but overall you will simply get better at doing that specific workout. and so you will also stagnate because all you are adapting to is that specific workout.
enter periodization ;). when someone completes workouts, their bodies will adapt and enable them to do that workout better. so say the workout is actually 4×200 at 90%, but your best 200 is 20.00 (more random examples). that means you should be running each 200 at 22.22. so as you complete this workout more often, you are better able to run 200s at 22.22. you will probably improve your max 200 time (or not, since this is a weird example), but only to a certain extent. so athletes stagnate because they are learning one skill (4×200 at 22.22) when trying to learn another (a lower 200 time). at this point, you start doing another skill with specific carryover to running the 200.
ok i hope i explained this well enough for you guys to get what im saying. i know its kind of a simplistic model, but i thought this was an easier way of looking at periodization for an average person. let me know what you think.