But whatever the specifics of any given quarter, the trend line is still clear: it’s going down. Which points to the same consistent truth: mobile devices have become the dominant computing platform.Why else would, for instance, Google upend how its search engine works to prioritize mobile-friendly sites? Google knows that to be useful, it needs to work best on the devices people are actually using. Meanwhile, PC-dependent incumbents like Intel—the primary supplier of microprocessors for less-than-mobile devices—has cited weakening demand from businesses for desktop computers and revised its revenue outlook down.These days, the big players—Google, Amazon, Apple, Microsoft, Facebook—are putting their resources toward optimizing their businesses for mobile. They haven’t all cracked the code, but they’re trying. In the meantime, PCs have yet to settle into a niche, given their diminished place in tech’s new hierarchy. With mobile, the question is, what else can we do with it? With PCs, the question is, what are they still good for?