Time, as always time is of the essence so have pre-built widgets for common elements makes a massive difference.

Creating a basic UI mock-up, to help our stakeholders literally see our vision, can take days to turn-around in our “UX” group, because they cannot see the value in sketch-up-quality mock-up that demonstrates the basic ideas. The widgets make it possible for non-Photoshop users to create viable, functional mock-ups in an afternoon.

I use the downloaded widgets to test areas with assumed user issues with by user testing. I do the whole process myself so don’t have time to build out every interaction, so any pre-built widgets that I can use in my scenarios helps.