Why should I use Struts? I don't mean comparison with other frameworks. When
I can use Servlets, JSPs, and POJOs to create MVC application, why should
use a framework? Is there any compelling reason besides easy?

Struts or other powerfull & component-based frameworks such as
JSF,Tapestry,etc provides many usefull features such as:

Web Components
Navigation Handling
LifeCycle Handling
Internationalizing
ans so on
In fact, they:

provide many reusable features
are in higher abstract level
increase system maintanability
decrease time-to-market
These possibilities are usefull in all projects specillay in enterprise or
large-scale system