It would be more weird if it was on the comics if they did that but in the forum it is mainly (I imagine) to drop some pounds of files from old threads that nobody looks at anymore. They also have it set up so that you can post images from the link address which bypasses that problem. Like my images in this thread are just "copy image address" from my stash over on deviantart so they will still appear even if a cache clear happens so anyone can use that method if they are worried about their images disappearing after a purge 
