The engines need to be overhauled and sometimes replaced several times over an aircraft's lifetime, so they are often swapped out and overhauled and then instal

The engines need to be overhauled and sometimes replaced several times over an aircraft's lifetime, so they are often swapped out and overhauled and then installed on another aircraft. Not only that, but some injection of new engines into the pool is needed. So airlines sign big contracts with engine manufacturers for new engines, parts, supplies, training, etc. This also happens even with small single engine airplanes, those engines need overhauls every so many thousand hours, so one option is to trade in your old engine for a refurbished one instead of overhauling and keeping the same engine. For some larger fleets companies have contracts with engineer manufacturers or service centers for this.