2 Different uses.

The proper FIA kill switch is designed to be used to stop the engine in the case of an accident. It pulls the alternator output to earth as the alternator output will still be connected to the rest of the wiring loom and therefore attempt to keep the engine running with the battery isolated.
It also breaks the supply to the ignition coil/ecu via another set of contacts for the same reason.

It also has another benefit. Normally, the battery acts as a sink for any voltage spikes etc. created by the alternator. These can be pretty bad for modern ECU's.
If you end up in the situation where the battery is isolated and the alternator still attempting to keep the engine running then you are in danger of doing some rather expensive damage.

So basically, Fit a simple isolator one if you like, but don't try to stop the engine with it.

Google FIA Kill switch wiring