The value here is that opposed to navigating back and forth between the new tab and Facebook, you can view comments, add new ones, and share the item yourself, all from one convenient navbar that hovers over the posted item. This should increase engagement around posted items, as well as give them a bit of a viral boost as users “re-share” items that their friends have posted.
Posted Items already became a bit more prominent on Facebook when they got their own tab on the homepage with the social network’s major redesign earlier this year. Some pure speculation: now that Facebook is further encouraging users to engage with Posted Items, could an aggregate view of the site’s most shared items (ala Digg) be in the works? It could be tricky given the privacy settings around user comments, but would certainly be feasible as the company expands its media strategy.