I guess it is case by case. I've tracked down a high number of people doing this, even down to spreadsheets with lists of all the logins they've created. What I've seen is creating profiles on sites powered by popular Content Management Systems, and using the toolbar PR of the homepage as the "value" of the link.
On the overwhelming majority of those sites, the user profile pages do not get a link unless the user has created content (there's no "user list"). Thus, the "clever" people doing this are then creating their own miniature "link farms" throwing a link at the profiles they've created so that they get the URL indexed, and, presumably, then charge their client for the link.