Studentweb is important for you who study at UiO. This is where you register as a student on courses (semester registration) and find payment information for the semester fee. In Studentweb you can also register for exams, and withdraw the exam registration again if you have to. You can change your telephone number and address information, order transcripts, submit complaint about grades and more.
In Studentweb, you can also order a semester receipt on paper, which shows that you are a student at UiO. But it is not at all certain that you need to do so, because your student card og your digital Student ID are more practical in most contexts.