Impersonating a user makes an administrator able to sign in to the system as a different user. This process is used to provide the administrator access to the modules and menus that the user had access to. ServiceNow records the administrator's activities when they are impersonating another user.
Impersonating a user is quite beneficial as it enables testing. We can use this feature for testing the solution without logging out from our session. If you don't use impersonating a user feature, you would have to log out from your session and log in again with that user's credentials for testing.
