Magneto check prior to shut down.
Why is it important to ground check the magneto's?
If you are asking about checking the magneto just prior to shutting down the engine, then here is an answer.
If the ignition switch is “off” and the magneto’s ground is open (malfunctioned) the magneto is electrically alive, and capable of delivering a spark, regardless of what position the ignition switch is in. The danger is that if somebody moves the propeller the magneto will send a spark to the cylinder, if you happen to have any fuel present it can (and has) start the engine.
By moving the ignition switch momentarily to off, and observing the engine beginning to shut down (due to a loss of ignition) you have verified the magneto’s wiring is intact Therefore you can be reasonably sure when you place the ignition switch to "off" the magneto is incapable of delivering a spark.