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The fundamental reason companies use commissions is because it shifts risk from the company to the employee. Companies that depend on salespeople tend to scale up with the number of feet you have on the ground. If you pay salaries, every employee increases your cost structure. If you depend on commissions, employees pay themselves or go hungry.

You can see this in an extreme form with car dealerships, where the salesmen are usually 100% commission. It's common for dealerships to have a half-dozen salesmen loitering on an empty lot, because they don't cost the dealership anything.