Since they all clue you in to be objective and reliable and observant, build on your skills and experience in those areas. Keep it tight, true and make certain that all grammar, punctuation and spellings are correct.
Be sure and get your reports in on time, paint an authenic picture of your shop, give details, read instructions and above all, be discreet. Look for the "question" required and answer it. The more you do, the easier it gets....good luck!
If I understand correctly you are asking how to answer the question on application forms that ask, "Why you believe you would be a valuable shopper?" As Mantis said, "tell the truth."
Why do you think you would be a good mystery shopper? Is it because you are observant, you pay attention to detail, you have a good memory, and you are objective? Pick out your personal qualities that you think will make you a good shopper. Depending on the space provided, give them examples of how your previous work experience required similar skills.