A tool to compare if 2 directories are the same and contain the exact same files in the same hierarchy.
There's plenty of choice and variety of tools similar to this, yet each has it's own uses and quirks. DDD was created with the primary intention of comparing massive directories of files (mainly pictures), supporting comparison on multiple machines.
No tool is perfect, and the world is not perfect, data corruption during syncs and backups will happen whether we like it or not. DDD can help you to find out what's different and where, while trying not to crash in the meantime.