How we can define correlation between rebound hammer and comprehensive strength? And why we need correlation between rebound hammer and comprehensive strength? If it is digital and show the strength of concrete.

There are predefined charts available to correlate rebound number with compressive strength , also we can define customize curves to obtain a relationship between rebound number and compressive strength.
Rebound number is required to calculate the compressive strength to verify the grade of concrete. Digital hammers are fed with predefined algorithms which directly converts the rebound value to compressive strength.