*This only works if you have keyless entry*
Say that you accidently locked your keys in your car. Your spouse has the spare set at home.
You have a few options:
1. Spend hours trying to open your car with a hangar and end up scratching the heck out of your windows
2. Pay someone to open your car
3. Tell her to come all the way across town to give you your keys
4. Call her, and she can unlock it at home
Now by "4" what I mean is you call her, and stand next to your car. She holds your extra set next to the mouthpiece on her phone, and you put your earpiece next to your car (even better if you have speakerphone). She hits unlock on the extra set...BAM your car opens.
Try it if you don't believe me, I did it today with perfect results. Just thought I would share this very valuable knowledge.