diffrential signal is compsoed of two signals out of phase these are called diffrential , these diffrential are imposed on a DC for biasing the MOS or whatever , this dc is called common mode
V1=Vd+Vcm>>>> V2=-Vd+Vcm
Vi/p diff.= V1-V2= 2Vd
Vi/p cm= (V1+V2)/2= Vcm
about the usage the signal itself is diffrentail which has many adv. over single-ended like noise immunity "common mode rejection"
about comon mode , it is not considered ur signal "dont carry information" but it can be used to bias the next stage or just generated from the previous stage "it can be removed , if not compatable with the next satge"
regards.