It's nothing new, the change has been several months in the making
The recent switch to the updated Google navbar came with an interesting addition, the share box is more prominently featured on most Google products, most notably on the homepage.This change isn't entirely new, but now that the final revision of the updated navbar is being rolled out to everyone, a lot more people are going to notice something new on the Google homepage, the share box.
Google has kept its homepage notoriously clutter free. Since its inception it featured a search box, the Google logo, a couple of search buttons and a few links to other Google products.
The new share box is hardly unnoticeable and that's the whole point. Google has made it very clear last year that social will be an integral part of Google and that Google+ is just Google evolved and not a stand-alone product.
It's still debatable how much of that is marketing talk and how much is true, but Google has done everything in its power to integrate social aspects into everything it does, even if it doesn't always make sense.
The share box featured on the homepage is just a continuation of that, granted, one of the most noticeable. But it's not that much of a change as others are making it, if only because the share box has been visible on the homepage for more than half a year, ever since Google+ debuted.
The only difference is that it was incorporated into the top navbar, making it a bit less noticeable, where as it is now part of the homepage. Elsewhere on Google, the share box is integrated in the unified header with the search box.
What's more, the navbar evolution introduced a couple of months ago, which did away with the top bar and integrated the functionality into the header also added the share box to the homepage in the same spot it's now.