Because:
A) I don't want or need multiple "copies" of a screenshot I'm trying to quickly crop extraneous junk out of to throw in a document
B) Because there is nothing inherently "wrong" with a destructive workflow - your bias against such a workflow suggests a "my way or the highway" approach... there are lots of ways to accomplish tasks which work for different needs and approaches.
A crop-to-selection option seems insanely basic, functionality wise. It should function as described in the earlier comment ... crops to the smallest bounding box around the selection.
I have a screen shot that includes a bunch of extra space around the window I want to focus on. Open in Photoshop, magic-wand select the stuff around the content I want to keep, invert selection, crop to selection, save. No copy and paste from clipboard... no multiple copies. Don't care that it was destructive. And the annoying thing about the idea of "just use the Crop tool" is that it doesn't have the magic-wand edge detection ability. It would require me to zoom in and drag the crop-bound-box to specifically make sure I cropped to the border of the window content I want to keep in my screen shot. Magic wand quickly selected against the board with no deep inspection on my part.
We don't all use Affinity tools just for perfecting a gallery photo... it is already open on my Mac, so why not use it for the mundane tasks? Well.. apparently I shouldn't use it for mundane tasks because it is not well suited to such basic workflows. Bummer.
I get that there are ways to accomplish my goals in Affinity photo, but really, as a programmer, crop to selection seems fairly easy to implement... so why not help out those who you are trying to woo away from Photoshop-type applications? You catch more flies with honey.
