The marks aren't as important as like say high school. Good marks might help you in applying for your first internship if that is relevant to your degree so that then you can gain experience. But you need to be well rounded i.e. experience + decent marks + other skills like working in groups, ability to present and speak well, so having that combination of things vs just good marks will be better.
It's not like you can just chill and study because those other things like doing a job whatever it might be to get some work experience and money for expenses, building up skills through volunteering or other workshops, require a lot of time to be dedicated to them. Due to that you might find you are more busy than you were in high school because you have to manage more responsibilities and decide which ones are higher priority, aside from the sole focus in school being to study and get a good mark.
For some degrees though the mark is actually important because for instance at UNSW if you don't attain a certain minimum mark you might not meet the requirements to do an honours year if that is something you want to do. If you want to do any future study as well like masters or PhD, then in this case the marks are very important because it will mean the difference between securing entry and a government scholarship or having to finance it yourself which is very expensive.