The use of an IP address (rather than even a username) is often a clever device that serves a multitude of purposes. An established username allows you to look at their other contributions and perhaps discern some clues as to their identity or affiliation. On the other hand, a relatively new account, if the user demonstrates experience/knowledge with Wikipedia, can provoke allegations of sockpuppetry. If the new user does not demonstrate experience/knowledge with Wikipedia, he is pretty much harmless as he will be ineffective in pushing his POV since he cannot game the system through his familiarity with the rules and norms.
An IP address provides the opportunity to disclaim responsibility for edits (since they could have been made by someone else using the same terminal) and, if the IP address shifts, can make it difficult to figure out which edits are being made by whom.