Cascading gives you the possibility to use downstream circuit breakers with less performance (Icc) if there is a coordination with the upstream breaker.
And less performance means lower price. See the following example.

Example of three level cascading

Consider three circuit breakers A, B and C connected in series.The criteria for cascading are fulfilled in the following two cases :

the upstream device A is coordinated for cascading with both devices B and C (even if the cascading criteria are not fulfilled between B and C). It is simply necessary to check that the combinations A+B and A+C have the required breaking capacity ; each pair of successive devices is coordinated, i.e. A with B and B with C (even if the cascading criteria are not fulfilled between A and C). It is simply necessary to check that the combinations A+B and B+C have the required breaking capacity.

The upstream breaker A is a NS250L (breaking capacity 150kA) for a prospective Isc of 80 kA across its output terminals. A NS100N (breaking capacity 25 kA) can be used for circuit breaker B for a prospective Isc of 50 kA across its output terminals, since the “reinforced” breaking capacity provided by cascading with the upstream NS250L is 150 kA.

A C60N (breaking capacity 10kA) can be used for circuit breaker C for a prospective Isc of 24 kA across its output terminals since the “reinforced” breaking capacity provided by cascading with the upstream NS250L is 30 kA.

Note that the “reinforced” breaking capacity of the C60N with the NS100N upstream is only 25 kA,but: 

• A + B = 150 kA;
• A + C = 30 kA.