why we are using set in java code example

Example 1: java how to make set

//Creating HashSet 
HashSet<String> set = new HashSet(); 

//adding elements  
set.add("One");    
set.add("Two");    

//Removing element);
set.remove("One"); 

//Removing all the elements available in the set  
set.clear();

Example 2: set in java

SET: Can only store unique values, 
     And does not maintain order
- HashSet can have null, order is not guaranteed
- LinkedHashSet can have null and keeps the order 
- TreeSet sorts the order and don't accept null