From Festipedia, hosted by the FR Heritage Group

When you are editing a page, a Show preview button appears below the edit textbox. You can click on this button at any time to see what the page will look like without actually saving the page. You will also see a preview of the edit summary. You are encouraged to check the preview before saving the page so that you can check how the page will look and make sure you haven't made any errors. It increases the chances of you getting your changes right first time, avoiding a series of minor edits to the page. Saving the page several times in quick succession clogs up the page history and makes it harder for other users to see what has changed. It also increases the chances of edit conflicts as some users watch recent changes and will immediately try to fix any factual or formatting errors in your text.

If you are in the middle of a large edit, you may want to copy your text to your clipboard before previewing it to make sure you don't lose anything.