App: A self-contained program or piece of software designed to fulfill a particular purpose (Google Definition). A smartphone would be nothing more than a paperweight if it didn't have any apps. The most basic function of a smartphone, i.e., making and receiving phone calls, wouldn't be possible without a phone app.
Apps are the lifeblood of a smartphone. Mobile phone and data plans generate billions of dollars of revenue each year for wireless communications companies. The phone, itself, is secondary, and, as such, is typically heavily discounted when phone and data plans are purchased.