Why cycle the magnetos before engine shutdown?
This is a hot-mag check to verify that when you shut the engine down, you are really putting it in a safe(r) state. Think about how the mag-switch works, when you switch to R or L, it grounds the P-Lead on the other side (so switching to R grounds the P-Lead on the Left side). If you don't see the RPM drop, you may have a hot mag on that side. 

An un-grounded mag is a dangerous situation in that it can cause the aircraft engine to turn over if you accidentally (or intentionally) move the prop when you are parking the aircraft, or when somebody walks by and runs into your prop. 

So during this mag check you are only looking for an RPM drop or a change that tells you that the mag is shut off.