Looks like nothing but branding to me, but I guess you could argue it provides some sort of consistency with the status bar. It might also be preparation for a more interactive lock-screen similar to what iOS 8 offers (http://9to5mac.files.wordpress.com/2014/06/img_00071.png).
The thing that bothers me about this is that the Apple logo is not positioned exactly in the same way as it is on the menu bar (it is shifted a bit on the left).
Looks like nothing but branding to me, but I guess you could argue it provides some sort of consistency with the status bar. It might also be preparation for a more interactive lock-screen similar to what iOS 8 offers (http://9to5mac.files.wordpress.com/2014/06/img_00071.png).