Grilles or No Grilles

Speakers with Grilles- on. With Grills- off.
Putting grills on some speakers sound almost like putting some cloth between you and the drivers.

My Frazier Concertos had similar foam grills. They were the closest to acoustically transparent as you will find.

....or maybe manufacturers provide the grills for aesthetics, so the drivers don't stare at you when you're not doing "serious listening" and better integrate with room decor........it's marketing.

Many manufacturers who supply grills with their speakers recommend listening with grills off, at least for "serious listening". I would venture that many feel it's a given that their customers will take them off.
Most box speakers, say 30 years ago, had recessed baffles and the grill frame and cloth would fit in to be flush with the front. Most did not have flush mounted drivers. Now, almost all speakers have at least flush mounted tweeters, flat baffles, sometimes beveled or rounded edges. Most grill frames are a compromise.
Ok, some designers spend great effort on the grill design and intend you to leave them on, but they are few.