Clothing retailer Gap launched a contest asking developers to create an iPhone app that promotes its brand, which I'm writing about this week. Yet all of this has got me thinking: What kinds of retailers should deliver an iPhone app in the first place, and which should not?There's an Amazon app, Pizza Hut app, Whole Foods app, Robert Andrews Salon & Spa app, among others. Many act as virtual mobile storefronts fully equipped with e-commerce to sell their wares, while offering some added value such as special discounts. All the apps tap into consumer brand loyalty and try to keep people interested in their products.[ Check out 10 great iPhone hobby apps, reports CIO.com. | Do iPhone app logos matter? You bet. ]"Some brands can leverage an iPhone app well," says Krishna Subramanian, founder of Mobclix, which operates a mobile ad exchange marketplace and co-sponsored Gap's iPhone app contest. "This year eBay made over $400 million off their iPhone app, and the app hasn't even been out the entire year."Retailers with customers who frequently make transactions are candidates for an iPhone app\u2014that is, there's a built-in reason to download and use the app. After all, it's important that there's a good reason to look at the app regularly; iPhone users tend to be picky about which apps are worthy enough to take up screen space and memory on their iPhones.Brands with a loyal following also should have an iPhone app, particularly when brands are tied to hobbies that help define a person. Grill masters, for instance, tend to be quite attached to their Weber grill. Hence, Weber's On the Grill iPhone app, which shows favorite recipes and has a grill timer and groceries list but doesn't sell Weber grills, has become a popular app on the App Store.On the other hand, there are brands that definitely should not have an app for that. An app that doesn't have a compelling reason for users to open and use it regularly would just be a waste of development dollars. "You wouldn't want an app if you were selling furniture," says Bill Westerman, principal and CTO of Create with Context, a design and research firm and one of the judges in the Gap contest.Speaking of the Gap, does it make sense for the clothing retailer to have an iPhone app? It's unlikely that there would be enough transactions to justify it. Really, how often does someone buy clothes? While Gap customers might be loyal to the brand, chances are they buy clothes from other brands, too. A Gap app is clearly on the bubble\u2014and Gap apparently knows it."We do not have plans to use the winning app at this time," Olivia Doyne, director of engagement marketing and public relations at the Gap, writes in an email.What's the dumbest iPhone app? Send me an email at firstname.lastname@example.org. Or follow me on Twitter @kaneshige. Follow everything from CIO.com on Twitter @CIOonline.