Hi Andy. I'm not quite sure what you're asking, but as Ros said the idea of the Preview button on the edit tab is to show what the biography will look like on the profile page when the edit is completed. So instead of showing headings likes Biography and Sources with equal signs, it shows them in a large bold typeface.
For the second part of your question, if you want to see what is in the raw text of the biography without opening the edit tab, you can use an app by Steven Harris called GetMarkup. You can get to it by using the Find menu and selecting Apps then scrolling down, or directly at this link:
https://apps.wikitree.com/apps/harris5439/getmarkup/
You need to enter the WikiTree ID of the profile you're interested in, then click the blue Get Profile Markup button.