The ability to use it when you don't want to run IDEA?

Sourcetrail can run on a server, and you can navigate a large codebase from a 10-year old laptop with little RAM. Or you might be using a different IDE for your embedded C codebase. Etc.

Traversing and displaying code as a graph is indeed the primary use case for Sourcetrail. It's documented at https://www.sourcetrail.com/documentation/ You can also click any part of the code to navigate the graph and vice versa. If you have enough code that this sounds useful, and run a supported language, you'll love it for what it does. And yes, I'm aware that, for Java especially, IDEA has a number of similar code navigation and code graph features. Try both and see which you prefer. :)