well. it sort of egg and chicken thing: you must have an IP and interface UP state to have an establish connection with anybody, right. so BGP takes that IP as ID. if u do not have any IPs or interfaces in UP state, you will not have any neighbors.
BGP router id process is similar to osfp one. the highest IP of interface in up state unless there is a loopback interface (highest IP of loopback). Manually specifying router id takes precedence over any automatic method.