Why is there as an option to log in as another user?

As an Admin user in Vtiger CRM, you can log in to the CRM as another user. 

There may be a few instances where you may need to log in as another user. For example, if a user is not able to figure out how to set up something or you want to personalize the CRM for them, you can log in to their account. 

But you must make sure you have their permission to do so.