Because all plate tectonics is driven by the crust floating on the molten mantle and you can't understand earthquakes unless you understand plate tectonics. Further more you couldn't understand the idea of Pangea and continent drift unless you knew about the earth layers. Then there are the Van Allen Belts; areas of high radiation. They exist because the earth's core is molten and spinning. The spinning heavily iron core creates a magnetic field and that creates magnetic lines of force. The solar wind is heavy radiation and solar particles that constantly bath our planet in intense radiation. Thanks to our magnetic field we are protected from this. The resulting radiation is trapped in the Van Allen Belts and that affects skin cancer rates in a good, or simply lessen the chances of skin cancer. How? Simple the amount of ultraviolet radiation that hits the earth is limited by the magnetic fields that surround it and the ozone layer,  so we have less skin cancer, less sunburns and the mutation rate is not too high to cause destruction to life. In some way all of science is important, we must know these things in order for us to have: a safer life, better understanding, and to know the truth about things to avoid certain disasters