End to end scenario using Webservic and SAP XI

Hi Experts,

I have read many blogs regarding Integrating XI using WebService, SOAP, HTTP for many scenarios but I am still confused and no blogs were clearly saying about the end to end scenario what happens where and how?

My question is could any one please please please explain me why we are using Webservice if you explain me with an CREDIT CHECK scenario using XI and Webservice that would be very fare.

Thanks in advance,

Cheers!!!!