Assessors are looking for a few things in a written exercise:
1. Can you actually speak English?
2. Reasonable grammar & spelling?
3. Can you structure thoughts coherently?
4. Can you distill your message down to its salient points?
5. Have you demonstrated that you actually have some content, rather than being all fluff?
Most native English speakers get points 1 & 2 correct. Quite a few people fall foul of rule 3 and ramble on, forgetting the golden rules of introduction, body, and summary.
Getting rule 4 right becomes more important as you become more senior in your career. I don't expect the same level of polish from a new consultant as I do from a senior manager. It is an excellent way to impress if you can do it.
Rule 5 is a tricky one -- you need to demonstrate that you actually understand whatever you're being asked to write about, without simply writing everything you know about the subject.