Sometimes when I edit a sentence and change words, I'll spot a letter or string, sometimes even short phrases, that can be fit into the new sentence, and instead of deleting the whole thing, I'll selectively delete only the part that needs to be changed. An easy example is leaving the "s" of a plural in place but changing the noun. Grabbing the mouse and highlighting takes more time than simply hitting backspace a few times, so it can't be a laziness thing.
Maybe I fear that my keyboard will run out of letters it can create. Oddly enough, I treat my browser as an infinite tab generator and throw them out with no qualms.