Question

why “data members” should be declared “protected”?what can be the possible benifits?

Answer 1

protected simply means that subclasses are able to see that member, but the member is not part of the public API to the object.

Re the question; it depends what you mean by “data members”. If you mean fields, then IMO they shouldn’t be. It is fairly common to make some state (properties) protected, if subclasses would need that info (in particular methods), but it isn’t necessary for the outside world.

A better example, however, is protected virtual, where the inheritor is able to change the implementation of an otherwise private (to the outside world) member. The classic example being:

protected virtual OnSomeMemberChanging(...) {}
protected virtual OnSomeMemberChanged(...) {}

where the inheritor can now react (or even block) changes to key values by using override to change the implementation (commonly but not always calling base.Whatever() at some point to invoke the original implementation as well).