They want to find out how much protection we humans might require. The other two instruments will search for signs of water and interesting minerals on Mars. Space radiation can be electromagnetic, like x-rays and gamma rays, or particulate, like protons and electrons. Particulate radiation is more threatening to humans. Cosmic ray nuclei, carrying typically 300 to 500 MeV per nucleon, travel through the human body so quickly there's not enough time to unload their energy into the surrounding tissue.
Solar protons passing through humans ionize molecules along their tracks.
Protons will sometimes modify or even break DNA strands within cells. If the cell survives it can become cancerous, which is a long-term health risk of radiation exposure. Mars' thin atmosphere provides minimal protection for the planet from energetic protons. Astronauts however, can find the protection within shelter walls made of lightweight materials. Future explorers however, won't want to spend all their time inside shelters, therefore, they'll need to know how to handle radiation levels outdoors.