The two main reasons are:
1. To give officers a little breathing space while preforming the traffic stop. Thanks to rubber neckers and drunks, a traffic stop is very dangerous for officers. When the officers is at the drivers side window talking to the person stopped, the officers car is offset, angled, and the wheels are turned to give the officer room to operate. The theory is if another car comes by that would strike the officer, the patrol car will absorb the impact and not the officer. The wheels are turned so that when the patrol car is struck, it will roll into the street and not become a second vehicle flying at the officer and the person detained. We also have to protect the people that are stopped from injury.
2. To serve as protection form the occupants of the vehicle the officer has stopped. If they were to start shooting at the officer, it gives him more protection by placing the engine block between the officer and the offender vehicle. Bullets have a more difficult time penetrating the engine block then they do glass, sheet metal, fiberglass, etc..
11 years Law Enforcement