Why cropping from the preview loses quality
A 6000-pixel-wide photo displayed in a browser is shown at perhaps 700 pixels. A tool that crops what it displays is working from that smaller copy, and the result is a fraction of the resolution you started with — usually noticed later, when the image is printed or opened full screen.
Storing the selection as a proportion of the image rather than as screen coordinates avoids that entirely. The crop is applied to the original at full size, so a quarter of a 6000-pixel image is 1500 pixels wide regardless of how the preview was scaled.
The pixel readout is there so you can check before downloading. If a platform needs 1080 by 1080 and your crop reads 840, you know before you upload rather than after it is rejected.