The question focuses on why you chose PT over health care careers, and I think that it is essential to focus on that. A lot of people write about wanting to help people or diagnose or cure to whatever, but any health care professional can do that. Make sure to distinguish why PT. Think about what sets PT apart from other careers; where in the timeline does PT fit in, and why do you want to be at this stage of the healing process? (prevention or improving fitness versus diagnosing and prescribing meds for example).
I think format can be anything that works for you, whether that is a structured 5 paragraph essay with an intro, middle and conclusion, or an essay that starts with an interesting story and goes from there. It's up to you, just make sure it is memorable 🙂
Disclaimer: I am an applicant just like you, so these are all just my opinions and things I've read/heard about from others haha.