What does it do apart from the call graph? If I had jump-to-definition and find-references in the line-by-line code review UI, what extra benefits would I have by using Haystack?
I think the main benefit of Haystack is that it allows you to keep a model of how the pull request works on the canvas, rather than manually needing to gain an understanding of it in your head.
Even with goto definition/references, I need to understand what parts of the pull request are net new and work together (e.g. I created a new function A that relies on a new class B and calls a new function C) and what parts are linked with existing parts of the codebase.
Normally, this would require me to do some digging around, but with Haystack a lot of this work is done for me in a visual way, allowing me to understand a pull request significantly faster.