The only real reason for me in the UK to join a union would be for legal representation, otherwise I can represent my own interests.
At least here in the UK our unions are heavily involved in politics - which is a massive issue. Currently, the leadership of the unions and the people in them are literally opposite sides of the political spectrum.
The Labour party was founded by unions, so it's no surprise they're still completely beholden to them.
https://en.wikipedia.org/wiki/History_of_the_Labour_Party_(U...
At any moment some change outside my control could occur and my place in society would change. Right now I'm pretty self sufficient and don't really need the support of others in day to day life, but that can change, and there's nothing I can do about it. Seems like a good idea to use this opportunity to try and improve things for everyone, even if you don't care about others, just in case your place in society changes. (I actually think it's neat if we try to improve things for everyone for everyone's sake tbh but I get there are people that do not have such empathy)