Confirmation and proof
Depending on maturity or novelty of the initial idea, the prototypes can be built very close to its production variant, or can be built to prove the concept. For the mature drone ideas, the purpose of prototype is to get quantitive confirmation of performance as well as discover and resolve integration issues, by doing number of specific tests on particular tasks.
For brand new ideas of drones, the main idea of prototype is the proof of concept and the type of tests are slightly different, such tests are more of qualitative nature.
Of course, during prototype buld and test we find the areas of improvement for futher refinements.