The types of environments that tend to move toward server-free are environments that have a very large data set behind an individual server. Very large is relative, but we're definitely using the word terabyte in a plural sense at this point, and the load on that individual server is too much. So, you want to move that IO load off the server and onto something else -- either a dedicated server or the disk hardware itself.
Check out the entire Server-free Backup FAQ.
This was first published in May 2008