Have you ever faced simple issue of security role after changing the business unit? Yes, you will. You will not be able to identify which security role was assign to a user whose Business unit has been changed.
This is a simple thing but if we have not noted down the security role of a user, we will not be able to identify the previous security role a user, as all the security roles gets removed.
If audit history for user is enabled. You can easily identify the user who has what security role.
So, as you can see in the above screen shot what security role this user has been assigned.
Hope this help, while changing BU so, this help to remember the user security role.