Use the NOT function to reverse a value or logical argument, that is, if logical is FALSE, NOT returns TRUE. If logical is TRUE, NOT returns FALSE.
Why would you want to do this? A common example is to reverse the behavior of another function. For example, If the cell A1 is blank, then the formula =ISBLANK(A1) will return TRUE. NOT can be used to reverse this result to FALSE like this:
In essence, by adding NOT, you are able to create a formula that behaves like ISNOTBLANK, which doesn't exist in Excel.