And you could dynamically and efficiently add/remove web apps and endpoints assuming your entire codebase is in C#.

You can also block the clients for a brief period while you make routing changes, or wait for any other arbitrary event to complete. This is one way to achieve zero-downtime upgrades, even in situations where the service is running as a single binary on a single host.

We use it to proxy request from "modern" part of application moved to .net core to legacy parts of application, that are yet to be converted.

This is a "premiere use case" we've seen people use YARP for.

I was assuming more the latter. Also benefit is you can extend and customize in C#.

You might need to deploy a single executable rather than multiple.