Imagine a scenario where you want to retreive records that you dont want. You can use the NOT operator to reverse the search condition.
For example , You want to get all the departments having the GroupName other than ‘Research and Development’.
SELECT * FROM HumanResources.Department where NOT ( GroupName = 'Research and Development' )
The other way of getting the same result is using the != in every condition.
SELECT * FROM HumanResources.Department where GroupName != 'Research and Development'