If you mean home video, it’s the only way to present the full image without cropping on a 16:9 monitor. Letterbox bars were a lot bigger in the old days on 4:3 analog tv though. I put up with a pretty small image on a 21 inch tv for years watching letterboxed movies on Laserdisc.

Cropping 2:35 films to fit a 16:9 image is commonly done for television now, and is almost as bad as pan and scan was in the old days. Part of the image is being chopped off.

IMAX screens are not as wide as a 2:35 film, but the screen is bigger and taller, so letterbox bars are necessary for most of the big Hollywood films that are shown.