The answer is: to determine whether we are excepting ourselves from a policy we think everyone else should follow
The universalization test is an indicator that tells whether an action is morally right or wrong. Basically, if everyone does the same course of action for a certain situation, it will be deemed as universal. According to Kant, what's universal is what is morally acceptable. Hence, when once is exempt from a policy that everyone follows, then he/she violates the universalizability.