Your Course Rep is there to represent the views of the students on your course to the staff, so their job is to make sure you are happy! They are also there to let you know what the University is doing for you and provide feedback from the meetings they attend. Without Course Reps, you may not hear about really positive changes being made to your course or University, or may miss the opportunity to give feedback on something you feel really strongly about.
Course Reps attend Staff Student Committee meetings, which are chaired by students. These meetings are the only formal forum that exists purely to hear student feedback, and without them there would be no way of keeping track of what students have said - so you may be annoyed by something that's been mentioned year after year, but there'd be no record of it. All students on your course should have access to the notes from these Staff Student Committee meetings so that you are kept up to date with what is happening in response to your feedback.
The Students' Union provides training to Course Reps to ensure they are fully prepared for the role. This includes making sure they know as many ways as possible of getting your feedback and giving you information. Your Course Rep might choose to speak to you face to face if they see you regularly, or send emails, or hold drop-in sessions on a particular topic. Make sure you know who they are and how to get in touch so that you can make your voice heard!