Odysseus must describe himself as a beggar because he wants the element of surprise on his side when he confronts the suitors. Remember that Odysseus has already been told that the suitors must die. His disguise is not something that he has just thrown together himself, Athena, the goddess of wisdom, is helping Odysseus. She is the one who told him that it would be wisest to go to Ithaca wearing a disguise. Athena is actually the one who cloaks Odysseus in his beggar disguise. She has not simply dressed him in rags, but she has made him look old and ragged. His face is covered in a graying beard and he has the appearance of an old man. In fact, in Book 16 Athena can hardly contain herself when father and son are so close to one another in Eumaeus' home that she beckons Odysseus outside where she proceeds to lift his disguise, changing him from young to old. When Telemachus lays eyes on the once old and tired beggar he is certain that it is a god because the transformation is so great. Athena replaces his disguise and Odysseus goes into his palace as the beggar and Penelope doesn't even recognize him, allowing him to hold fast to his element of surprise.
Odysseus disguises himself as a beggar in order to see what was happening with his wife and the suitors. He wanted to surprise them so that he could kill them without them expecting it.