For iOS and Android, this isn't about opening up the OS to using more RAM—the memory needs on a handheld remain negligible compared to desktop use. In fact, going x64 isn't a guarantee of better performance—plenty of Android 32-bit phones matched the initial 64-bit iPhone 5s. Plus, the first 64-bit Android phones, like the HTC Desire 510, didn't benefit at all by being stuck with an older 32-bit version of Android.
But smartphones going 64-bit has other benefits—things like data per cycle (and faster), better encryption, and overall moving to new 64-bit chips—specifically the ARMv8 architecture—with improved features, like power efficiency.
Ultimately, the 64-bit revolution is already here on PCs and smartphones. The marketing people don't even trump it anymore. You, the consumer, don't need to know much about it to be part of it.