Engines
Engines are made up of a complex series of churning, pumping, and rotating components, all grinding away at each other and powered by a contained internal explosion. It’s no big surprise that they should require regular maintenance. The idea of the full tune-up needing to be performed every six months is another one that no longer really applies.
Older automobiles relied on precision tuning to ensure that all of those engine components were working together the way that they were designed to. New cars on the other hand tend to rely on computer systems to keep everything in check. As such, tune-ups (beyond checking out belts, fluids, etc specified in the car manuals) really are not necessary.