If all users are using an application from the same device, the task is simple: provide an application that works great on that device. However, this is not common with web applications. Business applications might only concern themselves with desktop users, but even here it is likely that there is some range in screen resolutions and browsers being used. This is hardly new to web development, and the task is well understood. A single layout can be used without too much trouble.
Increasingly however, we need to also consider tablet and phone users. In the case of CRM Simple, we used a single page for all screens. The layout is simple and condensed enough to remain usable on a 7-inch tablet. If the screen gets much smaller than that, it becomes unusable. At the same time, while it may look overly simplistic, it's still usable on a desktop. Although phone screen sizes continue to drift upwards, they're still small enough to require significant simplification of the...