Are you heading into the outdoors or somewhere with limited cellphone reception? PLB's are also a good backup for if your cellphone gets wet/lost/has no battery. I personally keep mine in the glove box of the car when it's not out in a backpack with me, as often the NZ roads can have large areas with minimal or no reception.
PLB's don't require cellphone reception, they transmit digital signals to satellites which then provide your location to rescue services and send help your way as soon as possible.
If you'd like to use my beacon for your next adventure I just ask that you provide me with some info about your group and intentions so that in the unlikely event you do activate the beacon that I can pass this on.