Their purpose is to see if you have seriously thought about your future and to see if you can write. Spend time on writing them and show them to a USD faculty member for feedback. Be sure your application materials are complete and send them in. Sometimes financial aid applications are the same as admission forms, so you will not need to fill out no additional forms. But sometimes they are different, so fill them out with the same care you used on the application.
Financial aid comes in many forms.
Sometimes it may be an assistantship, either to help assist in teaching a course or in working on a research project with a faculty member. Sometimes it might be an outright fellowship or grant, but these are rare for new students. Some programs give not financial aid to first year grad students. If you prove yourself, then they are more generous. USD students have been very lucky in getting some kind of aid, but not all have received it right away.