In a word: Focus.
The key cost of accessing the web is that it is infinitely deep and wide. A person can get lost, or more accurately lose a lot of time when browsing, surfing or even precisely viewing specific sites. This is only a problem in web browsers.
In typical applications on your system, you have a very finite limit to what you can do that is not working. Once in a web browser, that limit disappears. So going to check your email or calendar can turn into a fascinating journey into the world wide web. Which is what your web browser is meant to be, your portal to the world wide web. It is not meant to be your portal to the world of Web-Apps. It does nothing to facilitate their usage.
Hana does allow you to work with limited distractions. And as Hana grows, look for features to grow that will facilitate getting your work done!