This addressing strategy is the oldest one used back in the days. Nowadays, we don’t work with it but lecturers are still teaching it to help students understand and analyse the addressing strategies.
Main idea of classful addressing is dividing IPs by certain subnet masks and create classes from them. There are five classes extracted by classful addressing and each of them has their own usages and attributes.
This is an old addressing and there was a lot of wastage of IPs. The idea was, if you were a company that needed 200 IP addresses, a class C assignment would have been provided. If you were a company that needed 50,000 IP addresses, a class B would have been provided. And if you were one of the few companies that justified requiring over 65k~ IP addresses, you would be given a Class A.