If you only have 1 key to your car, I would strongly advise that you get a spare one, no one wants to be left stranded or even not being able to get to work, kids to school and etc.
If you lock your keys inside the car having a spare means you dont have to call me out!
Getting a spare key cut from the main dealer can be very expensive 200-400 plus.
Having an extra car key is a wise precaution, for numerous reasons.
If keys are locked inside a car, the experience that follows can be extremely stressful. The time and expense of calling out a locksmith to retrieve them, or of having a window replaced if that's the route in you opt for, is something no one wants to suffer.
There are all sorts of scenarios that having a spare car key would allow you to avoid should you find yourself in this situation. Being late for or missing an important appointment, finding you've not only locked yourself out, but locked someone in the car - a pet or a child - it makes you see why the time you take to get yourself a spare car key is time well spent.
A spare car key will also act as a lifesaver if you've misplaced your original, or if you have a modern transponder type car key and it has stopped working.
Additionally, having a separate car key for another driver is a sensible idea as it saves you having to keep lending out your own. Providing you keep your spare car key in a safe place, it will serve you well should you need it.