The unions have a big presence on campus and a bar with heavily discounted drinks and organise a lot of the social clubs and events that happen outside of the study.
They also negotiate with the university and faculty, fighting for students, and you can get individual advise and help too.
Enrolling to the student union usual happens almost automatically when you turn up and register for your dorm, classes and doctors etc.
The staff of the university (academics, lecturers, trainers, instructors, researchers, etc) tend to belong to The University and College Union.[2]
I didn't know this was abnormal in other countries?
[1] https://www.nus.org.uk/ [2] https://www.ucu.org.uk/article/1685/About-UCU