couple reasons...

1) this lets you do actions async. one of my biggest peeves is that I usually remember to set the temperature right when I'm leaving, but as I'm heading out the door I have to keep the app open waiting for the car to wake up before I can turn on the AC (for example). now I just text my car "temp 68" as I'm heading out and it's nice when I get inside

2) there are some special actions like texting "marco", which will flash your lights and honk your horn to help find your car in a crowded lot, or texting "where are you" will send a google maps pin of the cars location

3) why not?