There's often some noise reduction (the plastic may be backed in foam) to quiet down the direct injectors on modern cars. There's also an element of less weath

There's often some noise reduction (the plastic may be backed in foam) to quiet down the direct injectors on modern cars. There's also an element of less weather-proofing required for the connectors underneath the plastic (or at least easier to undo/replace). It's also certainly about aesthetics when the car is being sold new - you open the hood and knowing nothing about cars (or modern cars) you see lots of little piece and become concerned that there's lots of things that could break. You open a car that says "$BRAND" in a $30 piece of plastic and you think, "Nice, this is a simple, reliable, easy-to-maintain vehicle!" There may be some small effect from highlighting the typical user service ports as well (windshield washer fluid, oil, brake, coolant reservoirs).