Why you need primary key?

The primary key of a table uniquely identifies each row in the table. It may also serve as a foreign key to other related tables. For example, a Customer table would use a CustomerID field (typically an autogenerated sequential integer) to uniquely identify each row in the table. This value would also appear in an Orders table as a foreign key to associate that customer with an order. Although unique individual values are best for primary keys, they may also be "compound" keys, that is, made up of two or more values in each row. For example, a table containing airline flight data might use a compound key of the flight number and the destination to uniquely identify the row.