Odysseus has been away for twenty years after angering the sea god Poseidon and undergoing a series of harrowing trials and adventures that test his wits and resourcefulness on his journey back to Attica. Now that he is finally back home, many things appear different to him. Twenty years is a long time, and he has no way of knowing what has happened in his household while he has been away.
The use of ruses and deception has been essential for Odysseus to overcome unfavorable odds and seemingly impossible challenges. He is not about to leave anything to chance by blowing his cover early before he has scouted out the circumstances. He needs to gather intelligence on whether his wife Penelope has been faithful to him, and whether his servants have remained faithful. This is better done under the cover of a disguise. That way he can freely observe and speak with many members of his household to ascertain the true situation. His throne and the stability of his kingdom are at stake, so he must get it right. That is why he chooses to hide his identity.
He discovers that Penelope has been faithful, but his household has been overrun by suitors who spend their time carousing with his food and wine. His household servants have been disloyal and aided the suitors in their quest for Penelope's hand in a gross violation of their duty to their absent master. Odysseus' hidden identity allows him to carefully sort the guilty from the faithful so that he can punish those that have taken advantage or betrayed him. This would have been difficult to do if he had revealed himself too soon.