Identity politics works for two reasons.
1) Self preservation. If you see a group of people running scared the other way, you are likely to join them as they may know something you do not. Same thing if a lot of people are telling you that you are wrong. Eventually you will start wondering if they know something you do not.
2) The idea that the other guy is so bad you better vote for me and not him. Remember when Biden stated, “If you have a problem figuring out whether you’re for me or Trump, then you ain’t black” Politicians use this all the time. Look at how much stuff they are throwing at President Trump hoping you will start to question if he is the right guy or not.
I would add a third. It is easier to convince a group of people to vote for you rather than convincing all the individuals to vote for you. Then you have the group basically campaigning for you.