What is Class diagram, why we need this?

Class diagrams are the blueprints of your system or subsystem. You can use class diagrams to model the objects that make up the system, to display the relationships between the objects, and to describe what those objects do and the services that they provide. Class diagrams are useful in many stages of system design.

Purpose Of Class Diagrams

Shows static structure of classifiers in a system
Diagram provides a basic notation for other structure diagrams prescribed by UML
Helpful for developers and other team members too
Business Analysts can use class diagrams to model systems from a business perspective

Basically Class Diagram is blueprint of your application and is show relationship of different component which are present in your application and subsystem.