Jason Kottke writes in the aftermath of the launch of Google’s Sidebar and IM application:
Aside from the browser and the Web server, applications will be written for the WebOS and won’t be specific to Windows, OS X, or Linux. This is also completely feasible, I think, for organizations like Google, Yahoo, Apple, Microsoft, or the Mozilla Foundation to make happen (more on this below).
Compared to “standalone” Web apps and desktop apps, applications developed for this hypothetical platform have some powerful advantages. Because they run in a Web browser, these applications are cross platform (assuming that whoever develops such a system develops the local Web server part of it for Windows, OS X, Linux, your mobile phone, etc.), just like Web apps such as Gmail, Basecamp, and Salesforce.com. You don’t need to be on a specific machine with a specific OS…you just need a browser + local Web server to access your favorite data and apps.
For application developers, the main advantage is that instead of writing two or more programs for multiple platforms (one for the Web, one for Windows, etc.), they can write one app that will run on any machine with the WebOS using the same code base.