Hello Maruti
This is a good question, and is important in order to understand the relationship between IGPs and BGP. Think about it this way. BGP is used to route traffic between AS’es while IGPs are used to route traffic within AS’es.
Now the next question that usually comes up is “since we’re using iBGP within an AS, why configure OSPF as well?”. Actually, iBGP within an AS is not used to route the traffic within the AS, but is used to share internal routes with the BGP routers on the edge of the AS so that they can advertise them with other BGP routers in other AS’es via eBGP.
So in order for iBGP to share these routes with all iBGP routers within an AS, routing capabilities between all BGP enabled routers within the AS via an IGP must be established.
Remember that an IGP is always better to use within an AS rather than BGP. You can find out more about why at the following post:
I hope this has been helpful!
Laz