They basically represent the interests of the students to the university. So if you have a problem with something within the university and your college is refusing to help you out you can take it to CUSU and they will support you, which might not seem all that relevant to you but is a pretty big deal to those who need it - and who knows, you might be one of them one day
They also try to make sure that the university and organisations within it act fairly to all students - for example, one of the issues at the CUSU council that I was at yesterday was to do with trying to make events and socials accessible to disabled students, and advertising those which were especially accessible. Again, not relevant to you, but important to somebody.
They also provide important training for JCR officers - I personally would not be able to fulfill my Access Officer job properly if it wasn't for the Access training and support meetings, and also just being able to email the CUSU Access Officer and ask advice. So anything done within your college by the JCR has probably been affected in some way by CUSU.
And they fund university societies. There are so many people involved in those that you can't possibly argue that they're completely irrelevant to university life
Aaannnd, plenty of other welfarey stuff.
I personally think it's good to have a centralised student body with the interests of the students completely in mind. But maybe that's just me