There are only a very few types of motion that act as transitions between techniques in combos. Your feet move as in ...
- Footsweep to control
- Turn the corner
- Step on the line or in the hole (with foot facing uke or facing away)
So, we teach all kihon in the form of combos, using those common motions - not because that's the most direct, practical way to get to each throws, but rather because it teaches the transitory footwork right from the beginning.
We also start everything from deashi - because it is what is common amongst all combos...
- Deashi-kouchi-(seoinage or seoiotoshi)
So, since just about any throw in judo is a minor variation of these nine techniques, and because you can prefix or postfix any throw in judo with deashi, by the time you get these nine throws down, you already know the prerequisites for all possible judo combos.
You could almost say that by brown belt you already know all possible judo combos - you just don't know that you know them. All you need re. combos is a system for organizing your thoughts about combination of techniques and a method for practicing and internalizing the ones that strike your fancy.