Hello Sourabh,
BGP is designed to interconnect different enterprises or organizations using different Autonomous Systems.
BGP provides scalability: actually the current internet could not be handled by any IGP: 300,000 routes!!
even ISIS cannot handle so many routes.
BGP has a rich toolset: BGP best path decision uses a hierarchy of criteria that goes beyond the calculation of a scalar metric. We could define BGP a protocol that uses a multi dimension vector/array of parameters as its metric.
For example think of BGP local preference or BGP weight : we can use this to prefer routes originated by different routers.
This allows for implementing complex policies that take in account multiple factors.
BGP is needed when there are multiple exit points to the internet and you want to control how these exit points are used.
Hope to help
Giuseppe
