MySQL 5.7 Release Notes
The primary key for a table represents the column or set of
columns that you use in your most vital queries. It has an
associated index, for fast query performance. Query performance
benefits from the NOT NULL optimization,
because it cannot include any NULL values.
With the InnoDB storage engine, the table
data is physically organized to do ultra-fast lookups and sorts
based on the primary key column or columns.
If your table is big and important, but does not have an obvious column or set of columns to use as a primary key, you might create a separate column with auto-increment values to use as the primary key. These unique IDs can serve as pointers to corresponding rows in other tables when you join tables using foreign keys.