Audit Viewer lets you browse your Mac's BSM audit records. The Mac's auditing system is like an airplane's flight recorder system for your computer. When something goes wrong, it can be the most valuable source of data to reconstruct exactly what happened and hopefully prevent a reoccurrence.
But like with an airplane, you want to turn on data recording before things go wrong. Turning on the flight recorder system after the plane crashes isn't very useful.
Audit Viewer is a forensics class tool. It is about digging down to the individual audit records. Unsure whether your audit system is configured correctly? Use Audit Viewer to verify that you can find the data you think should be there. Find some malware on your computer? Let Audit Viewer show you exactly what that malware did.
Apple's BSM system is incredibly powerful, but you need tools to unlock its power. Audit Viewer is one of those tools.