Maintaining your engine is very important, since you want to make sure you're changing the oil as recommended by the manufacturer, but that you also detail it a few times a year to protect the paint, rubber, and plastic pieces from fading or even worse, cracking over time.
All the rubber and plastic pieces take a real beating over the years because of the extreme heat in the engine bay and general wear and tear. By performing an engine bay detail, you can prevent costly repairs down the line, as well as give yourself a chance to inspect your engine's condition between maintenance cycles.
But let’s be real. If you’re reading this article, it means you have a real pride of ownership. You know you can get by with a dirty engine bay. You’ll still get from point A to point B just fine. Your re-sell value won’t even be all that much affected unless you have a classic. But you care. You want a clean, beautiful engine when you open up that hood.