In class declaration if we don’t write keyword “with sharing” then it runs in system mode then why keyword “without sharing” is introduced in apex

In class declaration if we don’t write keyword “with sharing” then it runs in system mode then why keyword “without sharing” is introduced in apex

Lets take example, there is classA declared using “with sharing” and it calls classB method. classB is not declared with any keyword then by default “with sharing” will be applied to that class because originating call is done through classA. To avoid this we have to explicitly define classB with keyword “without sharing”