German work contracts turn on a formal act called Abnahme, governed by § 640 BGB. Until it happens, the contractor must prove the work is free of defects. After it, the burden reverses and the client must prove the defect exists.
Acceptance also makes payment due, starts the warranty clock, and passes the risk of accidental damage to the client.
One declaration changes who has to prove what. Both sides know exactly which side of it they are standing on.