When making changes to the drivetrain of my gravel bike, I could not get it to index properly. I tried different derailleurs, different cassettes, double-checked cog spacing, changed cables, etc. I just could not get it to index properly at both ends of the cassette, at the same time. It was extremely frustrating, and many bad words were spoken. I bought a high-quality derailleur hanger alignment tool (Park DAG-3), and - despite it looking pretty close to straight to my eyes - I found that the alignment was actually quite a bit off. Proper alignment made ALL the difference, and smooth accurate shifting was the result. I adjusted the alignment of all my bikes, and every one of them shifted better immediately. It's not a tool that I use often, but I have used it again to re-set the hanger on my MTB after it got bumped pretty good on a ride. Having the right tool available is worth it to me. YMMV.