because you can work with individual microservices across clusters without a ton of overhead (or use a monolithic app server), aiding in deployment, rollback, debugging, development.
How exactly does having an app server reduce overhead, compared to running each service directly without app server?
And how does having an app server compare to putting each microservice in its own Docker container and orchestrating them in Kubernetes, which is what more and more companies seem to be doing?
having to deal with e.g. php-fpm, fcgi, tomcat, and unicorn separately in the same stack is a nightmare. even if they run in separate locations/clusters/nodes/machines, it's still several different configuration and deployment paradigms you have to deal with.
some people simply don't like containers or aren't tooled for it.
You would be able to merge your services under a single server and have them talk to each other internally sans latency overhead. It also allows you to easily scale up and down and segment things on demand.
At a glance, I think this is an alternative to docker/kubernetes. The general idea seems to be to cut the middleman/topman out and let the bottom man (app server) be the "unit" of configuration. Like a sort of integrated docker/<YourLang>-runtime.