Think, do example, about a dentist doing fillings on teeth, which is a service.
Because it is intangible it is not possible to measure each unit in the same way as we can with a product that is manufactured. Also, the fact that each filling will be different (some will take longer and some will be faster) i.e. they are heterogenous, we can’t have a standard cost for one filling as we can when we are manufacturing lots of identical units.