1. To update your bookmarks so that they are be the same on every computer/Kobo/Vox on your account.
2. So that you can buy and download books from Kobo without needing a computer.
3. To update your firmware.
4. So that you can share highlights, notes, Reading Awards and reading stats to FaceBook ( It's called Reading Life, the Vox and Kobo apps also have this), See this link for more
information about Reading Life.
You can also browse the web Via the built in web browser and download books from the dropbox website (The Mobile version doesn't work right, so go the desktop version).