Why we need to construct state transition diagram

Basically you need to construct a state transition diagram for each object with important behaviour. You need not construct one for anything with insignificant behaviour. 

The reason for doing this is to identify further attributes and operations, to check logical consistency of object and to more clearly specify its behaviour.  

Every best notation can be used to describe process they facilitate. 

Now we have basic concept of object models and dynamic models, our approach to analysis and design can be summarised as follow:

                                                                    Figure: Objects and State transitions