Why does Athena disguise herself as Mentor in the odyssey?

Athena disguises herself as a mortal so that may be treated as a mortal, giving advice to other men, as opposed to a god giving orders. In this way, she is better able to test mortals. It is also more fun for her, and it amuses her to become a mortal. It is a way for the gods to show their power, by behaving subtly.