1. Yes. If the schema group didn't exist you would have to make a pricing schema determination of each purchasing organization. This way, it takes less configuration entries to do the same thing.
2. Lets assume the configuration table for pricing schema only depended on the vendor schema instead of vendor + purchase org. Let's say I had schema Z1 for national vendors, and Z2 for international vendors. If purchasing org didn't belong to the determination of the pricing schema, I would only be able to have one pricing schema per vendor schema.