Fondair,
in simple terms:
Agplanes take stuff where people want it put.
Mixers/loaders make sure that the "stuff" is the correct product for that particular job etc.
Product can be liquid or solid(granular or powder)
They ensure the correct amount goes into the machine.
They make sure turnarounds are quick and efficient, allowing the machine to be out making money.
Spending time as a mixer/loader before becoming an ag-pilot, would and should be considered an essential part of becoming a well rounded operator.
Some may beg to differ, I am sure.
185.