To provide attribution by default.
In practice, it's implemented poorly. Usually I want to customize the citation format based on the type of document to which I plan to add the quotation. For example, if I'm quoting something on a Stack Exchange site, I want attribution using Stack Exchange Markdown. If I'm quoting something on All The Tropes, Wikipedia, a Wikia wiki, or my personal wiki using MediaWiki software, I want attribution using MediaWiki Cite.php markup. Users of TeX might want their own format, and they might want a quick way to switch among formats so that they can use one format with their scholarly articles and one on TeX Stack Exchange.
The ideal UI for automatic attribution would show a visible notice whenever the script inserts attribution, letting the user select the citation format. But Tynt and other providers of stand-alone attribution scripts tend to provide no user-visible way to customize the citation format, despite my open letter about the matter.