There are two other benefits to this, the first being the updated page view which lets you hop around the document a whole lot faster. The other is the built-in zoom, which scales the text to fit your monitor with a higher degree of detail than the text resizer found in your browser.
Gone with the transition is the option to open up PDFs as HTML pages straight from the message, which still remains as a viewing option once you're in the new PDF view and in search results from Google.com. Presumably Google.com results will get the updated viewer next. This would be a huge benefit to Google Search users without access to a PDF viewer (like on public or shared computers) as Google's current HTML conversion wipes out much of the formatting that can keep fonts and image placement intact, which can make things like brochures and newsletters unreadable.