Do you remember when IIS was the only reasonable choice as a web server for .NET. In fact I distinctly remember the only scenarios where the Kestrel web server was promoted was as a fairly trivial internal reverse proxy. Oh how things have changed!
Over the course of the last year the App Service Frontend fleet has transitioned from IIS running on HTTP.sys to Kestrel + YARP. This is a monster service and they are using Kestrel to drive it. How does Kestrel help? The biggest improvement appears to be around Performance and Cost, which includes decreased per-request CPU cost and per-connection memory cost. There are other improvements, of course, and the Azure and .NET team has a detailed post on the transition to Kestrel and YARP here.