WordPress has wrapped its app around the most recent browser version of the WordPress.com software. That’s it.
It runs well enough, but it doesn’t do anything that can’t be done in the browser. Moreover, there are some things it doesn’t do, so you are sent back to the browser version anyway.
There are only three reasons to use the app:
To keep Safari or another browser set aside for non-WordPress tasks.
To go straight to WordPress.com from the Dock or Application launcher.
If you want to store data locally on your Mac.
None of these are compelling:
WordPress.com works well in Safari. But even if you hate working that way. like it or not, there will be times when the app sends you there.
If you keep WordPress in your Safari bookmarks you can get there in two clicks instead of one.
Local data may help if you have a poor internet connection, otherwise, it’s rarely an issue. When I feel the need to compose a post outside of WordPress, I use a Markdown editor like iA Writer or Byword.
In short, there may be a case for people who spend all day managing WordPress sites to use the app, but for most people it’s just clutter.