To target the cancer. Immobilization of a body part during radiation is so that the targeted area ("field") can be precisely radiated. This avoids radiation to healthy tissues and gives the cancer as directed a treatment as possible.
To ensure accuracy. Radiation therapy works by focusing x-rays onto a precise area. Immobilization helps us better ensure that we deliver the intended dose to the exact location we planned on treating. If our patient moves during treatment, the therapy will be less effective and could cause harm to the surrounding tissues.