Unit 2: Changes to the "basket of goods"

The latest update of "in" and "out" items is in this BBC article

http://www.bbc.co.uk/news/business-12744400

and from Sky

http://news.sky.com/skynews/Home/Business/The-Basket-Of-Goods-And-Services-To-Calculate-Inflation-Updated-To-Include-Dating-Firm-Fees/Article/201103315952703?f=rss

and tutor2u

http://www.tutor2u.net/blog/index.php/gcse-economics/comments/iphone-in-shoulder-of-pork-out/

a) Why do the ONS have this annual update of the goods and services in the basket?