@trwnh I don't think there's any one answer to this

One of the most common use cases is commentary or response, a "loud reply" as @erincandescent put it. I think most of the hostile uses of quote posting fall into this category (which is not to say it's always or even usually hostile)

But also sometimes it's just to add emphasis, explanation, or even just personalization to a "boost." i.e., to share something with an explanation of why you're sharing