Hi Shreya, 

We introduced the reset test in case a learner gets disconnected, or has to pause in the middle of a section because it's time for bed or dinner time or something comes up, so they can to try again later.

To directly answer the question of why you can't undo a reset on a practice test if you change your mind, when you click "Reset" on a practice exam, a popup window appears asking if you are sure you want to reset, so this helps ensure that the learner is ready to take the next step and reset, which will involve resetting the scores, as well. Once we reset the scores, our system "forgets" about them, and allows the user a fresh start on the test. Once it "forgets" the scores, it can't "remember" them again, so that's why learners can't undo a reset.

Hope this gives some clarity! :)