I used Hypercard to design UI back then, UI was primitive compared to today, but the knowledge of how to design functional and useful UI was also very early. Hypercard made it easy to create interactive design you could actually interact with instead of just looking at.
Especially when the alternative was incredibly hard. Getting a simple UI running on the Mac involved using their IDE, learning a ton of Pascal, understanding the application framework, and memorizing dozens of Toolbox calls to get elements built and displayed.
HyperCard let you prototype the exact same thing in a matter of seconds.
