Why do I need to use foreign key if I can use WHERE?

Question

A beginners\' question about foreign key in MySQL.

In w3school it says,

A FOREIGN KEY in one table points to a PRIMARY KEY in another table.<

Answer 1

The primary purpose of a WHERE clause is to limit the rows returned by the query. See SELECT Syntax.

Primary key/Foreign key relationships maintain referential integrity and with proper indexing increase the performance of queries. (See Pete OHanlon's explanation, above and JOIN Types)