Hello Ylber
It’s important to note that the focus of this lesson is on creating BGP peers. In a topology such as the one shown at the end of the lesson, you have two links. Regardless of how user traffic traverses these links, your BGP peering must take place over those links. If you use physical interfaces on the routers as the source of the BGP peering, then the BGP peering will go down too, even though there is a second redundant link. So in essence, you lose the redundancy that a second link could provide to your BGP peerings.
By using loopbacks, and the multihop feature, you are sourcing the BGP peerings from loopbacks. If any single physical link fails, the BGP peering will still remain up. So the redundancy delivered in this case is to the BGP peering itself. It is a redundancy on the control plane.
Now once you establish this redundancy, you can then take care of using both links for user traffic, on the data plane. In order to do that with BGP, you can use features such as BGP Multipath and BGP Additional Paths, which appear in the following lessons:
I hope this has been helpful!
Laz