As is my wont, I like to explain things with analogies and this is no different. Consider a table holding job application details for a job board website.
CREATE TABLE dbo.tblJobApplications(
-- Application in progress
INSERT INTO dbo.tblJobApplicationsSELECT 1, 1, GETDATE(), NULL
-- Application unsuccessful
INSERT INTO dbo.tblJobApplicationsSELECT 2, 1, GETDATE(), 0
-- Application successful
INSERT INTO dbo.tblJobApplicationsSELECT 3, 1, GETDATE(), 1
The key field to note here is of course the BIT field which indicates the success or failure of the applicaiton. Obviously, when a candidate applies to a job, the success of the application isn't known - the candidate has been accepted, nor rejected. Its only at the end of the lifecycle of the application that this field can take on a meaningful value.
Hopefully, this contrived example helps explain just when you might require a NULL bit field.