Why bind MySQL tables with foreign keys if you still have the ability to select from multiple tables by key?

Question

MySQL

For example, I have a country (id, country) table and a cities (id, country_id, city) table.
I can say that the country_id in cities is the id in country, but what will it give if you can just leave everything as it is and query the selections from two tables in the same way? And linked tables, even when empty, take up more space. Explain, please.

Answer 1

https://en.wikipedia.org/wiki/Referential_Integrity

Answer 2

Thanks