It would save not one but two entire taps! Think of the milliseconds!
Especially when posting images, it’s nice to confirm you are indeed posting a meme and not a screenshot of your tax returns. And formatting can always get messed up once in a blue moon.
So, the existing flow is to write your comment/post, tap the three dots, tap preview, review your comment/post, tap done, and tap post.
The new flow would be to enable “preview by default“ in settings once. Then, write your comment/post, tap preview, review your comment/post, and tap post.
I definitely would. I’ve had to edit multiple posts after sending them because of formatting or other errors that were only obvious when viewing the post.