Not the most exciting reason for visiting a college, but I can tick this college off my list since I had supervisions at Sidney Sussex College for my first year maths module every week.
Supervisions are a key part of the Cambridge University education system. For science subjects, they generally involve two or three students and a supervisor (usually a fellow at the college, a PhD student or a researcher from the department) and last around an hour each. It’s a great opportunity to discuss any questions you have from the lecturers or your additional reading, and also to go over the answers to questions you were struggling with from the problem sheet (homework).
In the first couple of years it’s quite common to have supervisions in your college, but as you get more specialised or take more advanced courses they will usually be organised within your department and held there.