Do we need to know Earth's magnetic field locally? I'm a high school student and in our course we have elements of earth's magnetic field (dip and other stuff) that lets us know the value and direction of magnetic field at any point on the planet.
I just wondered why is this in high school course so I asked my teacher why we need to know about Earth's magnetic field that much( approx. value and direction at any point on the planet). Like, the only use that comes to my mind is studying how the magnetosphere protects us from radiation, but I don't think it requires us to know that much about it(or does it?). One historical use could be for navigation, but now it's all GPS based.
And my teacher replied sometimes that magnetic field interfere in our experiments or machines and so we need to take into account the disturbance caused by that.
Any examples, on how the magnetic field interfere, or some other reasons we need to know about it?
 A: Your teacher's reply is correct.
Generally, the Earth's magnetic field is very small compared to the ones we create for experimental purposes (I think its a few $\mu T$). However, many experiments use very sensitive magnetic field sensors to measure very small magnetic fields. So for example, if the magnitude of the magnetic field you are trying to measure is of similar magnitude to the Earth's, that means your sensor also senses "noise", which in that case is the magnetic field of the Earth. For this case, we'd like to remove from our data the measurement which corresponds to the Earth's magnetic field.
In a similar manner, if we are measuring very small charge displacements for example because of magnetic field or other physical properties influenced by magnetic fields, we should also include in our calculations the Earth's magnetic field, as this may also interfere with the final calculation for that physical property.
Have in mind also that magnetic field $\vec B$ is a vector physical quantity, which means we also want to know the direction of the Earth's magnetic field (and also the ones we measure).
Have a look at this, you might find it interesting: MEMS magnetic field sensor
