Mimicking another User can be very helpful when providing support and/or trouble shooting. The User may not be able to provide screen shots or explain what is not working. They will simply say "The website is broken."
When you mimic another User the Audit trail will assume that the User made the changes and interactions. The Audit trail is accurate, but can be misinterpreted. For example the date/time of the last logon for the User will change. Appropriate caution is recommended.
You can find the UserKey via Manage-Users. Typical is to set yourself and only today's date.