You know the problem: The hard disk is almost full, and you don't know, where the big files actually are lying, which occupy all the space. They may well be hidden in sub-sub-directories. To find out this by means of the Windows Explorer, is tiring: you would have to expand and collapse directories over and over or view the "Properties" of the directories, which always count just the size of a single subtree. It's hard to form a mental image of the hard disk usage in this way. This is where WinDirStat helps.