Why need of Foreign Key ?

When we have commands like join, and having clause to join two columns, then why we needy specialy a forigen key, and also tell the different between why a forigen key is used and why joining to two columns occurs 

The basic purpose of foreign key is to collect data from more than one tables. Many situations occur where data is required from more than two tables, like if you want to make a report of an Employees Salary. There would be a chance that you need data from more tables, here you will use joins.