Why two point starter used in dc series motor?

The only explanation to that is given by the presence of back emf Eb, which plays a critical role in governing the operation of the motor. The back emf, develops as the motor armature starts to rotate in presence of the magnetic field, by generating action and counters the supply voltage. This also essentially means, that the back emf at the starting is zero, and develops gradually as the motor gathers speed.

The general motor emf equation E = Eb + Ia.Ra,

at starting is modified to E = Ia.Ra as at starting Eb = 0.