Union Representatives are often referred to as Staff Representatives and that's no mistake. They do represent the interests of the staff, either collectively or individually depending on what's going on. But this has to be within the context of what the employer (legitimately, and in the interests of the business / organisation) has to do.
They are there primarily (I think) to hold employers to account, and to provide pastoral support to their employee members.
A good union representative will be very realistic with their member.
From my own experience, in individual circumstances, I would much rather a distressed individual be accompanied by a savvy union representative, since they can add perspective, and act as a supportive sounding board for the individual - than engage with their employer without that support.
Yes some are stuck in the 70s. I've been asked by some blatantly if I'm qualified to do my role. I've been on the receiving end of swearing, mindgames and abusive emails on occasion. But thankfully those representatives are a dwindling minority these days.