xkit-catalog.js is a script to allow you to self-host the Xkit Integration Catalog.
For background on how and why to use it, see the Self-hosted Catalog guide.
xkit-catalog.js is just xkit.js with some added features, namely a React app that serves as a standalone or embedded integration catalog.
That means that you can still use all of the functionality exposed by xkit.js, but you have a few more functions at your disposal as well.