Identity controls are a key function of web applications. You want to make sure that people are who they say they are, and that they can only do what they should be allowed to do.
Most, if not all web applications today, require some sort of authentication prior to using that app, so it is one of the first items that a person or hacker is presented with. Therefore, it is important that it works when it's supposed to, and denies users that shouldn't be there.
Testing your web applications for default usernames and passwords is an important part of identity testing across all your applications. Changing default usernames and passwords is a low-hanging fruit when it comes to a strong web application posture. You also want to make sure users have access to what they need based on their role. There shouldn't be 27 administrative privileged users...
