You see which selectors are used on the page:
and which not:
A look on the DOM tree:
Clicking on the pencil symbol, you can edit elements:
either change or add attributes
or the HTML content
Changes will have an effect immediately on your browser. So find out easily how style-changes would affect a page.
On Pages with a lot of style sheet definitions, you can easily find special tags or attributes on the style definitions.
E.g. you look for all font-size definitions:
You get a list with only those elements which fit the css attribute.