Question

why is it necessary to separate contact force into normal force and frictional force

Answer 1

The normal force is one component of the contact force between two objects, acting perpendicular to their interface. The frictional force is the other component; it is in a direction parallel to the plane of the interface between objects. Friction always acts to oppose any relative motion between surfaces.

Hence it is necessary to separate these two forces as they work in opposite direction in a same body and movement of a body depends on them.