Getting to preview your post write up before posting it cannot make you dictate your spelling error. I feel the preview button is more important when adding other tags (code, links, image etc) asides from just normal write up in order to get an actual glimpse of how your post will be when published.
Previewing your post before posting can allow you detect errors that you may have made while writing it. I usually will more easily see any typos when reading the preview of the post that when reading directly while posting.
It is also useful in double checking how forun tags will look before publishing the post.