When are GCSE grades looked at?

Question

what r they used 4?

Answer 1

To determine whether you're allowed into sixth form and which A levels you can study?

By universities, to see how you perform under exam conditions? (I don't think AS levels have an A* grade yet, so for the top universities seeing how many A*s a student achieved is a useful marker)

And, of course, if you don't do AS or A levels, they are the only qualifications you'll have to put on your CV when you are applying for jobs.