While sites like this may be intended as an additional research source when writing a big paper, it doesn't stop students at any level from just downloading a paper and turning it in as their own--or, in a word--plagiarizing. Most of the sites charge for their services; the one that doesn't also promises that it's work isn't high quality.
Some web sites are up to combat online plagiarism. Sites like turnitin.com and mydropbox.com deter plagiarism by having students submit their papers through the site which compares papers to web page matches and all papers previously submitted through their respective sites. These, too, charge the instructor or institution for their services.