Your Jeep is wired to only start if you are in Park or Neutral...
It determines this throught the Transmission Ranger Sensor...many call it the NSS or Neutral Safety Switch. Sometimes the switch goes bad, or gets dirty, and you can't start the Jeep...you turn the key, and nothing happens, because it thinks you are NOT in Neutral or Park. So, if you fake it out with the wire, your can get your Jeep started.
I don't advise leaving this wire permanently connected to ground, as someone can get hurt starting a vehicle when it is not in Park or Neutral.