Typically when an application is deployed on a server farm, it has a central database, one or more application servers and one or more web servers.
Since all the services in the application tier are going to be accessed via the web server, why not deploy the web server on the same physical tier as that of the application server?
The specific query is - what are the reasons of deploying the web server as a separate tier?
On a broader perspective, what are the general considerations while deciding on the deployment scenario?
If your application server is more CPU intensive than the Web server (which is the case most of the time), you may scale out the application server while keeping the number of Web servers as it is.
Put a NLB between the web farm and the application farm and you can have (for example) 2 web servers and 5 or more (or whatever) app servers to process the requests. It makes it easier to manage the infrastructure costs.