As far as I know, it started with the Maserati Bora in 1971. Which was a particularly idiotic move on their part, since the rear-mounted engine was under glass. You would look through the glass expecting to see the engine, and instead there was a black cover. I'm guessing they did it for noise emissions regulations, expecting their owners to immediately remove them after purchase.

The reason I think we see it on luxury vehicles is that it maybe creates 1-2 db of noise reduction (I have trained audiophile ears and I noticed no change in sound level at all), and snobbery dictates that their vehicles should not be composed of anything that could possibly get oily or greasy, so it's best to hide that stuff.

Also, from an automakers point of view it probably increases perceived reliability by a small amount, by keeping untrained hands from messing with stuff they shouldn't be touching. I've always assumed that's the reason they switched to sealed automatics as well. Motor oil and scraps of paper towel do not play well inside a transmission.

 

In my 2008 the engine covers are held in place with flange nuts, size M6-1.0. I think they _don't_ rattle off, but the shop techs can't be bothered to put them back in place. It's still better than my experience at US dealerships, where the vehicle was returned without engine covers, and the shop asserted the covers were missing as it rolled in. In any case, buy a batch on Amazon - that way they cost .10 to .20,, not the $1 at Home Depot or the $2 at the dealership.