Not everyone can handle or be available for a 40 hour workweek. Some of the best employees available may only be able to work for you as a second job or as a part-timer with flexible hours. Why lose a good person over following strict rules around a 40 hour workweek?
Whether its a student taking courses at night, a parent raising children afterschool or during daytime hours, offering people the ability to work 3-7 hour per shift instead of 8, or 3 or 4 10-hour shifts instead of 5 8-hour shifts, employers who provide flexibility can retain people who are still valuable.