Why is bgp needed?

Answer 1

BGP is the routing protocol used on the internet to exchange IP routes, and in order for routing on the Internet to work properly, all Internet-Speaking Routers must talk the same "language" (protocol), which currently is BGP (Border Gateway Protocol). So on the Internet, BGP is used to exchange public IP addresses and works as the glue of the Internet.

BGP is also sometimes used on internal networks, alongside technologies like MPLS etc.

Note that there are two kinds of BGP - eBGP (external BGP), and iBGP (internal BGP). Internal BGP is normally used within an AS - Autonomous System (usually, a company or an ISP), and eBGP peerings are used to fascilitate external Internet traffic exchange of IP routes.

So, the Internet is basically just a bunch of "bubbles" (Autonomous Systems), and eBGP peering is the mechanism that allows the bubbles to exchange information.