I don’t think it’s about bragging. If you’re "bragging" on your essays, you’ll probably not considered a very suitable candidate. It’s, like someone else said, an opportunity to see how you think. They don’t want you to be a good writer. You need to be able to express decently. What you express will matter. What matters is how you approach the essay prompts, how you share pieces of yourself and your personality, and basically how you add colour to your candidacy, which they know is more than just numbers and stats like test scores, employers, number of promotions, etc. It’s a beautiful opportunity to let the adcoms see you as a person holistically. It’s why interviews are important too. Would you say a person who is a better conversationalist or an extrovert but doesn’t know what he’s talking about have better chances than another who knows is ****?