Quotas and consequences
The primary purpose of imposing import quotas is to protect the domestic industry. The natural consequence of import quotas, therefore, is that domestic producers of a given product benefit from reduced competition, while international producers lose business.
Consider the example above. If U.S. consumers demand 50,000 German pianos per year, but supply is limited to just 30,000, then German producers are missing out on 20,000 piano sales per year. Presumably, that gap will be filled by U.S. producers.