I'm in a union. It's not recognised by my employer, but I see it a bit like insurance - I don't want to be in the position of having to use it, but if I ever am, I will be glad of it. And I find it an easy way to keep more or less abreast of changes in employment law and my rights, though obviously there are other ways of doing that. It seems many don't bother, though (like my line manager.)
I have been on a women in STEM day organised by my union, and I have had subsidised careers counselling through my union - it's not all about pay and workplace disputes.
I have used them for advice for a couple of situations over the years; I've also asked in MN, to get a range of opinions. For me, the union was there to say, yes, you would have a case, and these are the steps you'd need to take, and this is the evidence you nee to gather. MN was there for people to say, okay, you might have a case, but what do you want to achieve? I went through this, and I am not sure it was worth the effort, or I went through this, and it was horrible, but it paid off in the end, and I am in a far better position now. It gives a range of opinions, and that can help come to a decision about what might be the best way forward.