BGP and dual routers with dual ISPs

HI, I was reviewing Vincent Jones' High Availability book. He has one scenario where you have eBGP multihoming with dual ISPs. Each ISP has its own link and router. One example shows just using eBGP and running OSPF for the internal network but a later example uses eBGP and iBGP with OSPF. I don't understand the logic. TIA.