Because some of us use photos - croped and uncropped alike - to gain information about how close we can get to different animals at different places.
If I am only interested in shooting Greater Sage-grouse at frame-filling distances, and am looking for a new place to shoot them, and I use flickr to search out new places, and see a frame filling closeup of a Sage Grouse that another photographer posted, then I will want to know if the image was cropped, and if so how much it was cropped. . This information will tell me what I need to know about the distance that I can get to the subjects at that one particular place where the photographer shot that image at.
Many of us look at images on the internet as a way of doing research for future photoshoot locations. It's not just about looking at a picture for enjoyment. It's hard-core research and the attempt to garner as much information from every single photo we come across as we possibly can.
By the way, this need is not sudden. . At all.
Those of us who have used photo viewing for research, as I have described above, have always wanted to know how much images have been cropped. . This dates back even to the days of film and prints. . I remember asking Leonard Lee Rue how much one of his deer photos was cropped, and that was a photo I saw in a magazine way back in the 1980s.
Macro photography. Particularly with an MP-E 65. Specifying uncropped allows a size reference of the subject if magnification and camera type are included. With other macro lenses, 'at 1:1, or close to" also achieves this. In both cases if crop amount is described in a way that can be understood, again, size reference is achieved.
Landmark photos. Sydney Opera House is well known. I can say 35mm full frame with an 85mm lens will frame it with enough room to allow some horizon levelling from thr International Terminal. Or just post a photo saying 'uncropped' or 'slight crop' and convey the same info. For someone that English is not first language, they only need to undertand 1 word: crop.