MTBF answers "how reliable is this asset" in a single number — a shuttle with an MTBF of 2,000 hours fails, on average, once every 2,000 running hours. It only applies to repairable assets; for a component that gets replaced rather than fixed, the equivalent metric is MTTF.
MTBF trending downward across a fleet is usually the earliest, cleanest signal that a class of equipment is aging out of its useful life or that a maintenance interval has drifted too wide — long before OEE or throughput numbers show anything is wrong.
Where it fits the bathtub curve
MTBF is most useful during the "useful life" phase of the bathtub curve, where failures are roughly random rather than wear-driven. Applying a single MTBF figure across an asset's full lifespan — including infant-mortality and wear-out phases — understates near-term risk at both ends.