Because I want to be able to simply drag and drop an image for someone to see it immediately and not faff about with 15 extra steps of alt tabbing through browsers and copying links and waiting for uploads to finish.
There are several image hosts with tools available that make uploading as easy as passing a filename to a command or dropping it in a synced folder, so you're really just adding one step to the process of pasting an image into IRC.
Except I might not be working in a terminal, I might not have the file available locally (I personally don't have any stored on my computer). Like the poster above I'd like to drag and drop something from a tab to the conversation, and not have to deal with the steps between. That's what technology is about after all.