Primary Key : Concept

A relational database is based on the concept that the user that
queries the database needs not remember the physical location of
the record that he/she needs access to.

the user only needs to remember the table name and
the value that the user associates to.

that's why a relational database must contain a unique identifier
for each record that exists in the relational database.

thus the concept of primary key arises that provides a unique
identifier for each record called as the "PRIMARY KEY".