If you're new to Iroh, my mental model is roughly "Tailscale at the application layer instead of the network layer". If your question is, "why not just use Tailscale?", look at it from an app developer's perspective. If you want to release an app and have instances of your app be able to easily connect to each other, you could theoretically embeded Tailscale functionality into your app, but then the users of your app…
+ iroh and openziti can both be app-embedded
+ so the app developer embedding in their service is a good use case for both
+ openziti is used for services in which scale and security are critical
+ whereas iroh allows participation from parties which don't have any prior relationships - which can be very convenient