You want to stand out among the rest of the applicants, as well as provide new information that the admission officers do not already know about you. They will know what GPA you have before they start reading the essay as it’s one of the details they get with the application. Mentioning the GPA in your essay will sound like bragging and be uninteresting.
By zeroing in on one particular aspect of what is, invariably, a long story, you may be better able to extract meaning from the story. So instead of talking generally about playing percussion in the orchestra, hone in on a huge cymbal crash marking the climax of the piece. Or instead of trying to condense that two-week backpacking trip into a couple of paragraphs, tell your reader about waking up in a cold tent with a skiff of snow on it. The specificity of the story not only helps focus the reader’s attention, but also opens the door to deeper reflection on what the story means to you.