On an aspx page, this takes care of properly indenting all of your markup and ensures that everything is XHTML compliant (adds quotes to attributes, corrects capitalization, closes self-closing tags). I find that this makes it much easier to find mismatched tags and to make sure that my markup makes sense. If you don't like how it's indenting, you can control which tags go on their own line and how much space they get around them under Tools/Options/Text Editor/HTML/Format/Tag Specific Options.
In your C# or VB code, this will correct any capitalization or formatting issues that didn't get caught automatically.
For CSS files, you can choose compact (one definition per line), semi-expanded, or expanded (each rule on its own line); and you can choose how it handles capitalization.