we’re introducing today a new feature in Webmaster Tools - the Structured Data Dashboard. The Structured Data Dashboard has three views: site, item type and page-level.
Webmasters can use the Structured Data Dashboard to verify that Google is picking up new markup, as well as to detect problems with existing markup, for example monitor potential changes in instance counts during site redesigns.
Site-level view At the top level, the Structured Data Dashboard, which is under Optimization, aggregates this data (by root item type and vocabulary schema). Root item type means an item that is not an attribute of another on the same page.
Itemtype-level view It also provides per-page details for each item type.Google parses and stores a fixed number of pages for each site and item type. They are stored in decreasing order by the time in which they were crawled. We also keep all their structured data markup.
Page-level view we have a details page showing all attributes of every item type on the given page
Add an hcard microformat to your comments and webmaster tools will report number of hcards(comments) per url in a separate report including total number of hcard markups found across total number of pages. I like this.
Is that what they call it? I call it the Preview Everything Whether You Want To Or Not dashboard. Yes, OK, marginally useful to see that the site "linking" to you is just another search-results page. (I forget who in this Forum pointed out that that's the one thing Preview is good for.) But why in the ### would you need or want to see a preview of your own page?
For those who haven't stopped by WMT recently: Basically if you hover over anything they show you a preview.
Google captures all data they can, GWT is a way of letting us see some of what they collect and they tend to show things that we can suggest corrections for. This is likely just a way of showing us information they get anyway to allow us to see if our markup has issues or not.
It's beneficial all around, imo, but you aren't being forced to use it(until they figure out how to force G+ into it).