What is the earths magnetic field used for?

Good question, it is actually extremely important, this is because it protects the earth from harmful solar flares that would otherwise cause havoc with our electrical appliances. If you have heard of the northern lights and wondered how and why this happens then the reason is that the magnetic field acts the same as a bar magnet and the magnets force goes from the north to south poles and so this allows the solar flares to enter the atmosphere and when it enters the atmosphere it creates the northern lights.

Hope this way helpful in some way