This tool is for developers, who want to write (declarative) UIs or themes with instant visual feedback. A defined directory (e.g. your resources directory) will be watched for file changes and on write all connected browsers will refresh -- no reload, no compile, zero roundtrip.
While the Designer for sure is nice, once you know all the components from Vaadin by heart, clicking stuff together in a WYSIWYG editor is not every developers dream. Taking from awesome tools like ClojureScripts Figwheel, that give the developer instantaneous feedback, providing a tool, that
- allows for quickly testing UIs or ideas
- filling them with different mocked data (and keep them around for later)
- or just eliminate tiresome reload round trips to tackle some "why is the Panel in that TabSheet not scrolling" problems
increases developer productivity.