A primary key is unavoidable in an RDBMS. For a table, it enables each record to be unique without ambiguity. Indeed, it does not allow duplicates. Moreover it is indexed by default to improve search time. In a table CUSTOMERS example, the risk to differentiate each on his name is too big. Indeed, homonyms exist. Therefore it creates a field CODE_CLIENT where each code is unique in designating it as primary key to ensure that every client is different. Access will check this. It's like a file number, insurance, bank account or social security.