I don't quite understand where are you going with this particular thread, still let me try to answer what I do get from it.
Prey, as far as I know, is a tracking app for you Android, in case it gets stolen. Since you don't want to have the app on all the time (because, well, you still have your phone with you), they set the "send sms" option to active tracking: that way, if your phone gets stolen or lost, you send that sms from any other phone (it's not so much the number you send it from but the content of it, the key word) and it starts tracking.