Imagine you are a hiring manager or business owner, and job applicant is sitting across the desk from you. Perhaps you interviewed another job-seeker earlier in the day, who had some awesome credentials: graduated from a top university, great GPA, etc. But, he had seemed a bit complacent. The person sitting in front of you now, however, is a bit less comfortable, even a little nervous. You notice she is observing the contents of your office, and she is making an effort to maintain a smile. She really wants this to go well, and as you converse, you begin to understand she really wants this job. Her credentials aren't quite as impressive as the previous interviewee, but they are acceptable.
I don't know about you, but all other considerations being equal, I would likely hire #2. Why? She seems more motivated! And I want someone who will get things done. Solve problems. Motivate others.
This is not an employment website, of course. But many of the exams you may sit for are testing more than your knowledge on a particular topic--they are designed in part to test how you perform under pressure, and how far you can push yourself. Will you keep your cool during finals week?
To show your potential, for the vast majority of students, you will need to prepare, because the testing experience is so different from classwork and homework. The students who are motivated to take the time and effort to prepare will necessarily do better on the exams.