Why are force fields used to describe magnetic force?

The force fields are used to describe the movement of particles caused by a magnetic force.

Explanation

Magnetic forces can cause attraction or repulsion depending on the effect caused by the movement of particles.  

The magnetic force fields are imaginative lines surrounding the object where the magnetic force is applied.

The number of field lines determines the strength of the magnetic field and the tangent at the curve determines the direction of the field.

The direction of the magnetic force can be figured by right-hand thumb rule.

Like forces always repels while unlike forces always attracts.

explanation:

a fuel engine ( known as a gasoline engine in american english )is an internal combustion engine with spark ignition, designed to run on petrol (gasoline) and similar volatile fuels.