I took my car in for some work a while back. They had to replace a gasket on the engine because it was leaking a bit of oil. When they looked at the problem, they told me that I should replace a few other parts.
This was interesting because the parts were working and not causing any issues. The parts they wanted to replace were reasonably inexpensive, but in order to get to the failing gasket they had to spend a lot of time taking apart the engine. The time and labor to dismantle my engine outweighed the cost of the parts that they recommended replacing. It made sense to do this other work at the same time while they were easy to access. In the long run, this would save me money by not having to take apart the engine later when these components would eventually go bad.