User blog comment:LOLZJELLYFISHAREKEWL/HIYA BLOG FOR NOOBS XD/@comment-25112831-20151004124410/@comment-25428589-20151004130340

When you copy over a story from Word/another Word processing software, you generally copy the styles as well. You can't see this when copying, but the text is automatically wrapped in some HTML, which is what gives it the bold, italics, etc. When you view the page, you can't see these, however when editing the page in source mode (which some users do anyway, due to the extra control it provides) you can see these useless tags, which make no difference to the way the page, but add huge amounts to the page size.

TL; DR: For neatness and to speed up page load.