An undergraduate degree demonstrates to you what the characteristic activities of a field are like, and to observers that you can do those characteristic activities to some minimum standard.

Almost no one proceeds under the illusion that it's a comprehensive education. At best, you'll master a subfield in graduate school.

Continuous grading provides a valuable signal when you need to step up your game, transition to a different field, or drop out. I know several people whose poor CS grades sent them to tracks where they're now much happier and more successful.