The RAD environment makes it insanely quick and easy to throw together a reasonably functional desktop app. Think WinForms on steroids. For all its age and the many cracks and seams, it's still very fast, easy for beginners and pros alike, and the final executable is a single tiny file (usually)
I really find it mind blowing nothing else from what I've seen has really replicated it. Qt does an alright job but everything just seems to take 50% more effort to get it do the same.
I am very sad Kylix (the official Delphi for Linux) never went anywhere.