While the Culture is one of the most advanced and most powerful of all galactic civilizations, it is but one of the "high-level Involved" (called "Optimae" by some less advanced civilizations), the most powerful non-sublimed civilizations which mentor or control the others.
Although they lead a comfortable life within the Culture, many of its citizens feel a need to be useful and to belong to a society that does not merely exist for their own sake but that also helps improve the lot of sentient beings throughout the galaxy.