Why Co-ordination SPD is required? If we are provided service entrance SPD

As per the BS EN5 - Part4,  we have to provide SPD's (Surge protection devices) from the protection of transient induced overvoltage  (indirect lightning) to the internal equipment's.

If Incoming service line is overhead - Class I or Buried Class II SPD.

But why we have provide the co-ordination SPD for the internal equipment's. We are provide already at the incoming service lines for Class I or II.

If indirect lightning happened, service entrance SPD will protect to the system but what will do the co ordination SPD.