You'll find quite a few pilots have other forms of income other than the pilot salary. A number of pilots in my airline (F/Os and Captns) have fingers in other pies.
Often, those who have had previous careers can keep their hand in, and others go on to start other businesses.
For one thing it assists in the relatively low salary that pilots now take home, keeps up the standard of living, and second, it is always good to have another career if the airline you are in ceases to be.