Here is one way to explain to students why they take Track My Progress tests:

This test helps your teacher understand how to help everyone in your class learn what they need to make progress in school. You and your classmates have things that you are good at and things that you need help with. Track My Progress lets your teacher know what these things are so that all students in your class get the help and challenge they need. So don't worry about the test. Do your best, and if a test question is hard or confusing, your teacher will know that it is something you need help with.