Students should be assessed because there is no other way to find out whether they have learned anything.
Of course, assessments do not just have to be pen and paper tests. They can be done in a number of ways. Regardless of how they are done, they must be done. Much of the purpose of education is for students to learn things. If we do not assess them, we do not know if they have learned. If we do not know if they have learned, we do not know whether we have done our jobs properly and we do not know if they know the things that they will need in order to succeed as they go along in school and in life.