It finds duplicate files and replaces them with hard links, saving you space. Just make sure you provide it with paths in the same filesystem.
I originally wrote it to save some space from personal files (videos, photos, etc), but it turned out very useful for tar files, docker images, websites, and more. For example I maintain a tar file and a docker image with Kafka connectors which share many jar files. Using duphard I can save hundreds of megabytes, or even more than a gigabyte! For a documentation website with many copies of the same image (let's just say some static generators favor this practice for maintaining multiple versions), I can reduce the website size by 60%+, which then makes ssh copies, docker pulls, etc way faster speeding up deployment times.