Why did Athena disguise herself as Mentes in The Odyssey, Book 1?
It's often not a good idea for a mortal to come face to face with a god (the sight can strike people blind in other Greek myths, and sometimes kill them completely) so Athena's primary reason for disguising herself is to protect Telemachus.
Perhaps, though, we might view Athena's disguise metaphorically. If you were to say your dad gave you some good advice, and that advice came from God - it might be a bit like God coming to you in the shape of your dad. So it might be a reflection of Homer's pagan belief: that if Telemachus received good advice from Mentes, it was really the goddess acting through a mortal.
If mortals see gods in their true forms, something bad usually happens. They are usually blinded or burst into flames (like Semele does in the story of Dionysus's birth).
It is also important to remember that the gods of the Greek pantheon, the wise Athena included, are depicted as having very human motivations and emotions. That, combined with their divine powers, creates very interesting situations. Simply, Athena disguises herself so often (in the Odyssey, it is first as Mentes, an old family friend of Odysseus's) because she can.
She can come as ANY person or animal or even object, but she chooses Mentes because he represents loyalty and trust. It seems like it would be easier to convince Telemachus to do something as an old family friend, than it would be to visit in another shape and try to convince him that she is a goddess.