Do you wonder what your shop burden rates should be if you normally work 8 hour days but you only have 4 hours of work? Do you charge the customer for all 8 hours? There are some companies that will charge for 8 hours of work even when there was only 4 hours of labor. This gives an inflated cost to the product and may thus give it the appearance of being not profitable when if fact it was profitable. The unused time must be classified as idle time and its costs should not be attributed to a specific job cost or product family.
This unused time should be calculated and given to management in order to plan, schedule, and manage the company. Some situations when this may happen is when material is late, machine broke down, etc where management may be able to change processes to make the shop more effective.