Josiah,
The purpose of a data uri is to encode a web page or some part of a web page into a url. Thus, you don't need to have access to that external resource in order to display it. This is great for single page wikis like TiddlyWiki.
Example: Clicking on the following . . .(from the datauri example)
data:text/plain,some%20example%20textwill open a web page with the text in it. It will do this even if you are not connected to the web.
To pass the text of a tiddler to a URL use the datauri macro as in the example (Except use the title of your text tiddler instead of the jpg used in the example<$macrocall $name="datauri" title="Motovun Jack.jpg" $output="text/plain"/>