For the following reasons, magnetic flow lines are important:

1. The magnetic field lines shall be lines in a magnetic field, whose tangent shall give direction at every location of the field, and their density shall give field magnitude. 

2. They mark the magnetic field direction.

3. Intensity of the magnetic field varies on the number of lines. 

4. The lines at the poles are higher, thus the magnetic field at the poles is more powerful.

5. The force of the magnetic field depends on the number of magnetic field lines in a certain location.