I find it really handy for visually finding massive large files, for example that directory your video editing program uses to cashall its data before writing the final copy and never deleting its working files.
Its based heavily on the k4dirstat KDE program, but I find windirstat is far cleaner and less bloated than k4dirstat.
There is information for each section on mouse-over and you can click on any of the pie sections to redraw the pie using that as the 'root'. As far as I know, there is no other program for windows that presents this kind of UI (I have not read the other answers yet and I look forward to being proven wrong).
I did find a program like this with a more 'beautiful' UI but it was for Mac. It was years ago and I forget the name.
The best tool I have found so far is the oldest. It is called Disk Piecharter and was originally written for Windows 95 & 98. It still works on Windows 10. It's fast and easy to use. After you pick a disk drive to scan, it runs for a minute then presents a piechart that represents the top level in the disk directory tree. You can pick the pie slice you are interested in and it will redraw the piechart for the selected directory. If you want to go back up the directory tree, then pick outside the piechart. This way you can navigate right to the biggest folders that are of interest. IMHO, this is the quickest way to find the folders that are eating the lions share of disk space.
The program is a stand alone exe. The only problem... this application is no longer supported, but I don't care because it works fine as is.