Unsold inventory will depreciate in value, and having lots of unsold inventory can be devastating to any business. Apple has done an extraordinary job of having a high inventory turnover and having very low days of inventory on hand. This explains why Apple iPhones are always out of stock on the day of release. The people at Apple probably projected the number of sales in the first week, and carefully factored in the worst case scenarios. Thanks to their impeccable projections, their inventory rarely depreciates in value. Companies like Apple, use a variety of metrics to monitoring the effectiveness of their processes.

Metrics are an important part of any business for several reasons. What do the numbers in the table above tell us? Firstly, metrics are often quantitative, and thus gives an objective look at how a company is doing. Secondly, metrics are standardized: They are used throughout the industry and thus, they share the same meaning. Inventory turnover rate for Company X will mean exactly the same as inventory turnover rate for Company Y. Lastly, metrics allow people to compare different businesses and gauge how successful a business has been doing in a certain year. Companies certainly use metrics for very good reasons.

Source: http://www.fool.com/investing/general/2012/03/23/apple-lesson-of-the-day-inventory-is-evil.aspx