I can guess a few things. For one, its slightly less effort than selecting the chosen text and copying, though not much less so I don’t think that’s the whole reason. But also, it does technically give more information that otherwise would have to be typed out, like its often possible to tell what platform the text comes from, the username and profile icon of the original poster, etc. And if there’s a chain of several people responding, you can capture the whole conversation without worrying about formatting nested quotes and such. Fonts can also convey mood, which you’d lose if you just copy the text.

Finally, I guess it could feel more clear as to the source, like if you copy paste text with some quote formatting, there’s more room for someone who doesn’t understand the formatting to think you were saying that thing, and it’s immediately obvious that you could edit or make up the quote if you wanted. A picture immediately says “this is something this other person somewhere else said” in a way that’s harder to misinterpret and looks harder to fake (even if in reality making a faked picture of text is easy)