What does it actually change?
Structure first: it puts the main point up front and the reasons after it. Then sentence level: run-ons get split, filler like “basically” and “in order to” goes, repeated words get varied. It does not invent facts or add a greeting and sign-off you did not write.
Improve writing or Correct grammar?
Correct grammar fixes mistakes and leaves everything else alone, which is what you want for a text that is already fine. Improve writing rewrites, so use it when the draft is technically correct but hard to read. If you only need it shorter, Make it shorter is more predictable than a full rewrite.
What about text messages?
For messages, use Improve my text. It keeps the result conversational, sticks to the meaning and lets you set who you are writing to and the style. Improve writing is tuned for longer prose: emails, posts, reports, cover letters, anything with more than a couple of sentences.
Where it works
On the TextAI keyboard as Improve. Select the paragraph in Mail, Notes, Slack or a browser form, tap Improve and the rewrite replaces it in place. You can also share a screenshot of a draft to the app.