In judo, deashibarai (and/or kosotogari) is the connective tissue for combinations, because...
- anytime you are in a grappling situation (close enough to touch) you are likely to be able to reach deashibarai/kosotogari
- anytime you try a deashi/kosoto and it fails, it leaves you in synch with the other guy (better off than you were)
- you can do a deashi-anything combo
- you can do an anything-deashi combo
- this means that you can pretty much insert deashi in the middle of an anything-anything combo as a productive bridge between thing-1 and thing-2. Or in other words, deashi allows you to connect any two techniques in either order into a combo.
For these reasons, just like our judo pretty much starts and ends with deashi, our study of combinations starts and ends with deashi.