Type a width or a height and the other dimension follows from the ratio you have chosen, so images and video stay proportional. Useful for resizing exports, sizing embeds, and checking that a crop will not letterbox.
16:9 is the standard for landscape video on YouTube and television. 9:16 is vertical for Shorts, Reels and TikTok. 1:1 square and 4:5 portrait perform well in social feeds.
It is the marketing name for ultrawide displays, which are usually 64:27 (about 2.37:1) rather than exactly 21:9. Cinema widescreen is 2.39:1, which is close enough that films fill an ultrawide screen with only a thin border.