Random, fortuitous, haphazard, undirected, unplanned, and most importantly, unpredictable. There are already many ways to discover great content that you know you like, but how do you find things that you cannot begin to describe?
The majority of researchers begin their search for content using a general purpose search engine (Ithaka S+R | Jisc | RLUK: UK Survey of Academics 2012 [PDF]). It is easy to forget just how phenomenally powerful these can be, leading researchers to content that they know they want. This is also its shortcoming. The normal mode of searching makes it very difficult to find things that are not known yet. Keyword searches do not make it easy to collide ideas and concepts together, and to view things from different perspectives and to see what might fit together.
While many of the major providers have made attempts to provide related content to search results, these fall short of being serendipitous. The idea of searching for content fails when the researcher does not even know what they might want to see or how to describe it in words. The Mechanical Curator approaches discovery from the opposite angle, publishing content as it sees fit without an outside agent directing what it should publish.