Crash reporting tools like Crashlytics, Bugsense, HockeyApp, Crittercism do exactly what they’re made for. They will report any kind of crash that happens by sending you the stack trace. But nothing else!
What if the crash was caused by corrupted data or unexpected behavior? This would never show up in the stack trace. The only way to debug this kind of issue would be to connect the device to your development machine and go through the logs. Bugfender does exactly this, but without the need of having the device right next to you. You can debug the error while your customer is happily playing a game, browsing the web or even while using your application.
In summary, Bugfender is able to collect both logs and crashes, giving you a complete solution for diagnostics while your application is under testing or already live in the stores.