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It's for both your safety and the officer's safety. If you are in your car the officer knows exactly where you are and you aren't exposed to traffic. A suspect outside the vehicle can physically assault the officer much more easily than if he's seated in the car. The officer can observe for indications of a gun etc. as he is approaching the car. If he's trying to keep an eye on traffic and the suspect outside the car he might miss something. A lot of agencies are now telling their officers to approach on the curb side of the vehicle rather than from the traffic side.

When stopped stay in your car, keep your hands on the steering wheel if nighttime turn on the dome light and wait for instructions from the officer. Don't go digging for your wallet, insurance card etc. until asked for them and tell the officer what you are doing when you reach for your documents. Don't get pepper sprayed, tasered or shot because you made some sudden move that was perceived as a threat.

That's what I learned at the Citizen's Police Academy I went to.