There is an EMF ( voltage difference ) between two conductive surfaces.
Between those there is an electric field. By definition this is the force on a charge.
The electric field is affected by the distance and by the geometry of the surface.
At any point if the electric field becomes stronger than the force binding electrons to their atoms then the atoms become ionized and hence conductive. So the current follows that path.
This in turn alters the electric field in the space around so that once started it increases the field.
This is like tearing paper or plastic. It can be difficult to get it started but, once started, it takes little force to continue the tear.
So a sharp pointed object has a high field near it. Even if the distance is greater and the average field is lower, the current will leave that sharp point first.
This is why, in your car,, the new spark plug has a sharp edged electrode. So the sparks leave that edge easily.
But that means most sparks leave the edge wearing it more than the rest.
The spark plug now gets rounded corners and it takes a higher emf to cause a spark.
So you find that in some conditions your wires fail or your spark coil cannot make sufficient volts and the car fails to start. Yet with a new plug it starts easily.