why use both web servers and application servers in the same architecture?

Question

I understand the difference between the two, but I don't understand why you would use both. What is the point of separating the work between the two types of servers and increasing the amount of network calls.

Answer 1

Lots of applications use loads of flat files, even those containing scripts to be executed by the browser, are flat files for the web server. These files can be delivered much faster by a web server than by an application server busy executing backend logic, database requests, ...

There are more network calls, but most of those between the webserver (acting as a reverse proxy) and the application server are local, perhaps even local to the machine, perhaps using persistent connections.