Last week House of Fraser announced that it was going to offer free O2 wifi in 11 of its stores across the country. This followed announcements in recent weeks from Ted Baker and John Lewis about the imminent roll out of wifi across their store networks.So why is wifi such a priority for retailers at the moment? Put simply, it's because mobile shopping is growing so fast.The etail trade body IMRG reported that mobile sales had grown almost ten-fold from 0.4% at the beginning of 2010 to 3.3% in May/June of this year. But mobile commerce is only part of the story - the number of customers using their smartphones to support their shopping experience is even higher. IMRG and eDigital Research reported that 1 in 4 customers had used their smartphone to research a purchase when in store.So if customers are already using their smartphones in store, using the existing 3G signal, why should retailers be offering wifi?Firstly, 3G signals are notoriously unreliable indoors. By offering wifi, retailers can position themselves as helping customers to do what they want to do with their smartphones in-store without being hampered by variable coverage and download speeds.Secondly, it enables retailers to capture more data on their customers. For example, the service being rolled out in John Lewis stores is free to use as long as customers register in advance and surrender their email address. In addition, once customers are logged on to the system, stores can track exactly where and how they're using their phones - invaluable information in planning in-store mobile experiences of the future and a key reason behind House of Fraser's roll out.And finally, the received opinion is that shoppers able to use their smartphones to support purchases offline are more likely to make that purchase. The speed and convenience wifi offers smartphone shopper to check online reviews, use branded apps and check inventory for products that may not be available in store helps to support the purchase process and enables purchases that otherwise might not have happened.Of course, the big fear of retailers is that wifi will make it more likely that shoppers will find cheaper alternatives elsewhere. But the belief is that shoppers are still more likely to complete their purchases in store even if they find a product cheaper elsewhere. This is likely partly due to convenience, partly because the price differential becomes a known quantity rather than something the shopper has to leave the store to find out and partly because the retailer has created a positive impression by facilitating the comparison in the first place. In fact, for brands such as John Lewis, facilitating price comparison actually strengthens their proposition of being 'Never Knowingly Undersold'.And in the future, wifi connected customers in-store present retailers with interesting opportunities to use their customers' phones as mobile point of sales - be that promotions for products checked out online or via targeted push messages to opted in customers dependent on where they are in-store.Given the opportunities that abound for retailers in this field and the edge it could give them over rivals in an increasingly competitive market, the question shouldn't really be 'why wifi' but, frankly, 'why not'?