Lets say we have two different applications which are part of bigger system so they have the same set of users. If it is some closed system you will probably want to restrict some groups of users from accessing one o the applications (ie. client didn't buy that feature so his users shouldn't use it). If you wanted to use the standard credentials system implemented in the symfony, you'd have to check in each action if user have a proper credential. In our system we have hundreds of actions so it'd be a real nightmare. Especially you'd have to do it for both apps.
