Brand Autopsy has an interesting discussion on category killers:

Costco has a very loyal clientele (who pay a membership fee). Costco understands the taste level of this group and caters to their wants and needs–for wine, apparel, technology, jewelry, etc. Costco communicates with those customers through an excellent monthly magazine, the Costco Connection. Because many of Costco’s customers are small business owners, the magazine (and, of course, the stores) cater to their needs and interests.

Now, Costco’s customers are also attracted by the deals. There is an implicit understanding that Costco is offering the best possible price on that particular product on that particular day. The product may not be offered tomorrow. So, if you want it, you better buy it today.