Your bass guitar is a machine. I has several parts that all need to work together within. certain tolerances. However, your bass is largely made of wood, and wood is a natural substance whose qualities may vary. Not only this, but it can and will react to environmental changes like temperature and humidity. This means that even though your bass had a setup done on it before it left the factory, things may change over time and that setup will need to be revisited from time to time.