Tumblr doesn't have any problem driving up traffic, but it's operating in a very crowded space and, much like everyone else, navigating a migration to mobile, so any little bit helps.
The latest addition is a brand new compose window, or pop-up rather, that makes it even easier to post to Tumblr.
Not that it was ever hard, that was the whole point of Tumblr when it first came about, to simplify blogging. Still, up until now, writing a new post, sharing an image or a link, took several steps and had the user go through a couple of pages.
Now, a new post can be added directly from the dashboard, clicking on the buttons to write a new post or add a video will open up an overlay compose box with everything you need, on top of your dashboard, rather similar to how Twitter does it.