Why is a primary-foreign key relation required when we can join without it?

Question

If we can get data from two tables without having primary and foreign key relation, then why we need this rule? Can you please explain me clearly, with suitable example? It\

Answer 1

A primary key is not required. A foreign key is not required either. You can construct a query joining two tables on any column you wish as long as the datatypes either match or are converted to match. No relationship needs to explicitly exist.

To do this you use an outer join:

select tablea.code, tablea.name, tableb.location from tablea left outer join 
tableb on tablea.code = tableb.code

join with out relation

SQL join