Why do we need to have a backbone AREA 0 and why do we need to mention AREA ID as "0" for a backbone AREA?
nope, just ospf must have a backbone area. it's loop prevention and design requirement, see https://learningnetwork.cisco.com/thread/2201
RFC 2328 on ospf syas "The OSPF backbone is the special OSPF Area 0 (often written as Area 0.0.0.0, since OSPF Area ID's are typically formatted as IP addresses). The OSPF backbone always contains all area border routers. The backbone is responsible for distributing routing information between non-backbone areas. The backbone must be contiguous. However, it need not be physically contiguous; backbone connectivity can be established/maintained through the configuration of virtual links.
.... In OSPF domains the area topology is restricted so that there must be a backbone area (area 0) and all other areas must have either physical or virtual connections to the backbone. The reason for this star-like topology is that OSPF inter-area routing uses the distance-vector approach and a strict area hierarchy permits avoidance of the "counting to infinity" problem. OSPF prevents inter-area routing loops by implementing a split-horizon mechanism, allowing ABRs to inject into the backbone only Summary-LSAs derived from the intra-area routes, and limiting ABRs' SPF calculation to consider only Summary-LSAs in the backbone area's link-state database.