This doesn't add anything directly - instead this adds a networking layer for mods to communicate data more easily. Generally the only reason player's should download this is if a newer update is released that contains bugfixes, or if a smaller mod doesn't include this in it's jar file.
Full developer information for using this library (as well as dependency instructions) can be found on the wiki, and JavaDocs can be found here: https://alexiil.uk/javadoc/