The total radiation exposure you get while going through the outer van Allen belts is something like 0.16 to 1.16 rads for reference it takes 1000 rads to get a lethal radiation dose. The main reason to do this is to test the radiation effects on the new computer systems as the transistors are far smaller than what was used on the Apollo missions, a charged particle could flip a bit inside the computer and cause faulty information to be output if the shielding for the computers isn’t good enough.