An Application Pool can contain one or more applications 
and allows us to configure a level of isolation between 
different Web applications. For example, if you want to 
isolate all the Web applications running in the same 
computer, you can do this by creating a separate 
application pool for every Web application and placing them 
in their corresponding application pool. Because each 
application pool runs in its own worker process, errors in 
one application pool will not affect the applications 
running in other application pools. Deploying applications 
in application pools is a primary advantage of running IIS 
6.0 in worker process isolation mode because you can 
customize the application pools to achieve the degree of 
application isolation that you need.

We can simply say using application pools we can delegate
separate worker processes to separate applications.