Why does Athena disguise Odysseus as a beggar?

Athena disguised Odysseus as a beggar in order to protect him from potential harm and to ensure that he would be treated kindly by the people he encountered. By appearing as a poor and helpless stranger, Odysseus was able to gain the sympathy of the Phaeacians and receive the help he needed to continue his journey. Despite Antinous' attempt to intimidate him, Odysseus was able to remain composed and use his wit to gain the support of the other suitors. This allowed him to remain safe and continue his journey without any further harm. Additionally, by disguising himself as a beggar, Odysseus was able to observe the suitors and gain insight into their character, which would prove useful in his eventual confrontation with them. He was also able to use his story of misfortune, which included his time spent with the goddess Calypso, to gain sympathy from the suitors, which allowed him to receive food and other resources from them. This enabled him to survive and continue his journey without any further harm. Furthermore, Odysseus' disguise as a beggar allowed him to gain the trust of Menelaus, who was able to provide him with information about his father's fate. 

Here are the top passages from The Odyssey related to the question:

To this Nausicaa answered, “Stranger, you appear to be a sensible, well-disposed person. There is no accounting for luck; Jove gives prosperity to rich and poor just as he chooses, so you must take what he has seen fit to send you, and make the best of it. Now, however, that you have come to this our country, you shall not want for clothes nor for anything else that a foreigner in distress may reasonably look for. I will show you the way to the town, and will tell you the name of our people; we are called Phaeacians, and I am daughter to Alcinous, in whom the whole power of the state is vested.”

Then Antinous said, “What god can have sent such a pestilence to plague us during our dinner? Get out, into the open part of the court,[145] or I will give you Egypt and Cyprus over again for your insolence and importunity; you have begged of all the others, and they have given you lavishly, for they have abundance round them, and it is easy to be free with other people’s property when there is plenty of it.” On this Ulysses began to move off, and said, “Your looks, my fine sir, are better than your breeding; if you were in your own house you would not spare a poor man so much as a pinch of salt, for though you are in another man’s, and surrounded with abundance, you cannot find it in you to give him even a piece of bread.”