Why didn’t Odysseus tell the Phaiankians who he was immediately? Was there a benefit in hiding his identity?

He was ordered to conceal his identity until guaranteed safe passage home. When he had duly impressed the Phaeacians, and they had promised to help him return to his home, he revealed his identity and told his story.

Yes, there was a benefit. Had they known who he was from the beginning, he might not have even been allowed to enter the castle.