The above code is related with a hotel booking screen. When you want to review your booking details the state of the flow will be changed to the reviewBooking view state.

And the transitions above are mapped to other view states.

Web flow...

Spring Web flow is just like a session but behavior is bit different

What is a spring web-flow?

Many web applications require the same sequence of steps to execute in different contexts.
Often these sequences are merely components of a larger task the user is trying to accomplish.
Such a reusable sequence is called a flow.
Consider a typical shopping cart application.User registration,login,and cart checkout are all
examples of flows that can beinvoked from several places in this type of application.
Spring Web Flow is the module of Spring for implementing flows.The Web Flow engine plugs
into the Spring Web MVC platform and provides declarative flow definition language.

Why we use Spring core for software developments? Do we nessary it?

No. But Spring core provide a reconfigurable xml which supports for change objects dependencies externally without bulk code changes. That's the main advantage of using spring core.