Often they are used to replicate data. Case in point is the [TerminationDate] of an employee. That column will be NULL for everyone who is still working and have a valid date for folks who have, or were, terminated. Therefore a [Terminated] bit column is not needed.
I would regard this as overloading the [TerminationDate] column by giving it two meanings (terminated yes or no and date thereof). In addition to a column on the main table that states the current status, I would generally have a table with a foreign key of the main table and two columns of 'status' and 'date/time of status' (maybe others such as 'who set the status', etc.) so I could view all changes of status in chronological order, thus allowing for a status going to one value and then to another. For the employee, I could then see the dates of actions such as 'hired', 'suspended', 'terminated', 'rehired'. I can add new status by allowing new values in the 'status' column on the subsidiary table rather than having to add new columns for each new status to the main table.