I can see why Plant has to be assigned to a Sales organisation because Plant becomes a valuation area for a (one only) Sales Org. (entry in T001K where BWKEY is Plant)
We also assign Distribution Channel to Sales Organisation. (entry in TVKOV)
If we have assigned Plant and Distribution Channel to Sales Org separately, then what is achieved by the third step of assigning Sales Org + Dist Channel + Plant? (entry in TVKWZ)
I am guessing the answer lies in the fact that we can have multiple plants and multiple Dist Channels assigned to a single Sales Org and a need to restrict (or allow) permutations on the three. i.e. Allowing sales-processing for specific combinations of Distribution Channel and Plant.