I am seeing a tab in soap client as attachment. What is the use of that?
The "Attachment" tab in the SOAP Client and Message Responder/Stub tools is only used for sending requests to SOAP services that implement the SwA (SOAP with Attachments) specification. This tab lets you configure the MIME/DIME attachments expected by the SOAP service. However, please note that SwA is not common anymore as most web services implement the newer MTOM (Message Transmission Optimization Mechanism) specification. In other words, the "Attachment" tab generally isn't used anymore. More detail:
Working with Attachments