Retailers reap a number of benefits by getting shoppers to download their mobile apps, Mulpuru said. For one, they encourage loyalty and repeat purchases among consumers; for another, the apps that are well-executed elevate the customer experience. She cited Starbucks' incorporation of its payment technology and loyalty program as a prime example.
Apps are also essential in executing on the future rollout of beacon technology, which uses location devices in stores to push notifications and relevant promotions to shoppers. For beacons to work, they require shoppers to download—and opt into—an app.
Read MoreWant to predict retail earnings? Use this
For many companies, however, the benefits of having a mobile app aren't worth the costs to build and maintain it. Julie Ask, also a vice president and principal analyst at Forrester, said that on top of the $2 million up-front fee for building an app, it costs another 80 percent of that fee each year for maintenance—per platform.
That's a big chunk of change considering it only costs about $200,000 to build a mobile-optimized site, Ask said. What's more, the investment can seem particularly hefty given that, on the high end, only about 20 to 30 percent of a retailer's mobile sales come from their app, Mulpuru said.
As a result, it tends to be the larger retailers that are willing to shell out big bucks for the technology.