In the current society, every one has a profession which provides her money and resources for survival. For a person having enough amount of money or other resources may consider the profession for survival as optional. But it is not the case for all.
This may be the same in all earlier yugas. Some had to teach, some had to be in the army etc., You can observe that there is a concept of working hours in current days. A person does her work related to her profession during her working hours and may be engaged in other activities in the remaining time.
Is there any such concept of work time in the scriptures talking about previous three yugas?