Because university teachers can't explain every exercise. If you can't, you can ask your teacher and classmates. The teacher should arrange a question and answer time.
However, if you have done a lot of questions and still can't do them after thinking about it, then look at the problem analysis for reference, which saves time than around the discussion. But don't rely on solving problems, because some methods are wrong or bad, and relying too much on the answers is not good for your study.
So whether it is necessary to buy exercise solutions depends on personal circumstances (by the way, you can go to the school library, because the library may have books on the analysis of exercise answers)