Let me start with two corrections to the question, that can hopefully also help to answer it!
- Aircraft are usually ‘pushed’ by ground vehicles known as tow vehicles such as this:
The reason I stress pushed is that it is used mainly to reverse an airplane from the terminal area until it is in a position to move by itself.
2. These tow trucks will only help move the aircraft to an area where the pilots can safely switch on the engines, not the runway! (Ground vehicles are rarely allowed to move on a runway (save for emergencies and inspection), and the aircraft is supposed to move to the runway by itself)
With those out of the way, let us come to why the aircraft needs to push back in the first place:
Aircrafts don’t have powered wheels, and hence cannot move in reverse (let alone forward by themselves). Imagine a tiny jet strapped to one of your toy cars; that’s essentially how airplanes work on the ground! They go where the engines push them towards. (Note: There is a mechanism known as reverse thrusters, which allow the engine airflow to be pushed towards the front, but that is a highly risky maneuver and is only used to reduce speed immediately after landing.)
Another reason to push back an aircraft to a safe area is for safety reasons. The engines produce tremendous thrust from the exit, and an immense vacuum from the front; enough to suck in people, small vehicles, ground equipment etc (edit: Stefan Gebhardt shared a great example in the comments).
Hence it is a common procedure to start the engines once it is cleared away from the terminal area and lined up on a taxiway (roads that lead to the runway).
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