Let’s answer that question straight up front: you don’t. Not today, where nothing exists that’ll take advantage of the 64-bit architecture in Apple’s A7 processor — not in a meaningful way.
Apple’s rightly claiming it has the first phone-based 64-bit chip, but it’s also future-proofing — a little like noting a TV supports UHDTV despite the absence of 2160p (3840 by 2160 pixels at 16:9) content. At some point that content shows up, but in the meantime, you’re paying for a promise. And in the iPhone 5S’s case, you’ll want to weigh the device’s other assets before making a decision.
On the other hand, there’s what Apple’s saying and what the media’s saying Apple’s saying about the iPhone 5S, and rarely do the twain sync up. Apple’s as guilty as any company of hyping the holy living you-know-what out of its ideas, but that’s no reason to make the critical mistake of throwing every claim under the bus.