Why should we allow only single session per user?

Some time back me , Mishra and Mittal were discussing some technical issues and then Mishra mentioned that he had a requirement to implement this feature to allow only one session per user. He implemented this without any problem but then we started thinking what are the possible reasons for which people want to implement this requirement. After some thinking, discussing and googling we could come with below reasons:

1. Security. If we allow only one session per user then on creation of second session we can alert the user that there is already a active session and allow a way to kill the previous session. This way if the user is unaware of previous session then this will serve as a warning that his/her credentials may have been compromised. And user can change his credentials.
2. Licensing. Some products are priced as per number of users using the product. So, avoiding the multiple session per user will prevent the misuse of the license.
3. Product Implementation. This is very specific to the product requirement. If the application maintains some kind of user's working state then multiple sessions can mess it up.

 I will update this post if I could find some more reasons.

~Manish