A full screen capture shows far more than you intended. Cropping is as much about what you remove as what you keep.
Tab titles are the usual culprit. A screenshot taken to show a single paragraph also shows everything else you had open, and those titles are often more revealing than the content you meant to share.
Notification banners are worse, because they appear without warning at the moment of capture and contain message previews. It is worth scanning the top of the image before you send it anywhere.
Cropping removes those pixels entirely rather than covering them. That distinction matters: a black box drawn on top can sometimes be undone, while cropped away pixels are simply gone from the saved file.
No. Cropping discards pixels around the edge and leaves the rest untouched.
Yes, crop to the region you want and then resize to the pixel dimensions you need.
No, which is worth knowing given how much screenshots tend to contain.