why we use unique key in sql code example

Example 1: what is unique key in sql

Unique Key:
Only unique value and also can contain NULL

Example 2: primary key and unique key in sql

What is the difference between UNIQUE and PRIMARY KEY constraints? 
There should be only one PRIMARY KEY 
in a table whereas there can be
any number of UNIQUE Keys.
PRIMARY KEY doesn’t allow NULL
values whereas Unique key allows NULL values.