Because well-written content is one of the cornerstones of most search engine optimisation strategies, demand for content has grown exponentially alongside the web.
Naturally, this has led to the practice of "stealing" content. Sometimes this takes the form of a wholesale copying of content. On other occasions, this means the subtle rewriting of content so that the meaning and intent remain identical, but the actual words are different.
To the search engines this posed a problem - albeit a relatively minor one. If they were hoping to serve good quality content to searchers, how would they distinguish original content from mere facsimiles?
The technical problem this posed was actually relatively trivial: because the search engines maintain comprehensive indexes of most websites they can easily ascertain which version of copied content appeared first and disregard the later version.
As their linguistic understanding increases, they are also more able to determine content which matches existing content but is merely reworded.
The accuracy of their duplicate content detection scripts is such that today, duplicated content can incur penalties for the site guilty of stealing content.