Let's step back a minute and look at why they ask you to do this. They don't need to hear how passionate you are about being a nurse, how you always wanted to be a nurse because your sainted mother/auntie/grandma was one, because you got good care from one when you/your child/your grandma/your auntie/your mother was sick, you want to be a missionary/disaster relief/CRNA/military nurse to take care of refugees/our military heroes/the poor, or how sure you are that you will be a great nurse because of your passion/empathy/drive to be excellent, or any of that. They see so many of those that they all tend to run together. That's not to say you can't write about those things, if that's what sincerely moves you. However, because pretty much everyone writes something along those lines, what do you think will make your essay stand out?
I'll tell you what: literacy. They want to see if you can write an essay, which, by the way, is defined as: "a short literary composition on a particular theme or subject, usually in prose and generally analytic, speculative, or interpretative." They want to see that you can spell and use words correctly, avoiding flowery, convoluted, or "academic" language. They want to see that your sentences are well-structured, with subjects matching verbs and well-placed modifiers. They want to see that you can size up an idea, look at the evidence about it, and analyze or interpret it. Never use a four-syllable word when a one-syllable one will do.
Don't roll your eyes at this. Do not think for a minute that this is of little or no importance as to whether you'll be a good, caring nurse. As the ads say, if caring were enough, anyone could be a nurse. You will need these skills to read, interpret, and communicate data in your education and your practice after graduation. Want your essay to persuade them to admit you? Demonstrate how you have the tools to succeed at it.