Experts, I was asked a question recently that how can a Set<Id> be used to limit SOQL search results ?

Set<String> names = new Set<String>();
names.add('Test');
names.add('Test2');
System.debug([SELECT Name FROM Account WHERE Name IN: names]);

SET<String> cnt = new SET<String>();
for(Contact c : [Select Email From Contact]) {
         cnt.add(c.Email);
}
List<Contact> contactsWithValidEmail = [Select Email From Contact Where Email IN: cnt];