Hello Sudheer,
The Sap has the PI system (Process Integration), where you can create a channel to communicate with external systems and external systems can communicate with SAP.
In your case, to call the .NET Web Service, is necessary do the configuration in PI system and after this step, generate the Proxy class.
The Proxy class, is a class that communicate with PI and the PI call the DOT Net web service.
The real scenario to creating and consuming Web Services is an integration between SAP and third softwares, for example:
The company has a Web Application that is responsible to create Sales Order. Using a webservice, is possible, send the Sales order created on Web Application to SAP ECC.
I Hope this help you.