There are a few reasons that come to mind. Much of this depends on what your app does. Online banking? You want to log more than a community forum with no sensitive information or monetary value. Business app vs public?
1) IP address the user logged in from. This would provide some information should the account be compromised, and could help track down the offending person.
2) Determine if the user was able to login after failed login attempts. For instance, 9 failed attempts followed by a successful one means something very different than 9 failed and then no successful login.
3) Determine when the user last logged in, for various reasons, including whether they are actively using their account. 5 years since the last login? Not an active user.
4) If this is a business app, then you do want to log whether somebody accessed the application or not. For instance, if we know Bob is out of the country but then access the application, that could indicate something fishy. "Something fishy" will entirely depend on the application and your business.
Those are a few things that come to mind. Personally from my experience, I have used successful logins numerous times to help with internal and external investigations.