By Christopher Mims 

It's important to understand that like the iPhone before it, the Apple Watch isn't at all what its name would imply. Let's call it what it is: a wrist-top computer.

Just as smartphones have become supercomputers in our pockets, the Apple Watch and its many competitors, including Android-powered devices from Motorola and Samsung Electronics, are poised to become something more. And it's their central place in a larger ecosystem of apps and hardware, rather than any one thing that has been shown off recently, that will make them indispensable.

"There's so much [the Apple Watch] can do that we haven't really even talked about," Tim Cook told USA Today on the day of the announcement.

When a company like Google or Apple deliberately creates a place for other companies to sell their own, complementary wares, it's called a platform. Apple, more than Google, has mastered this art, having worked hard to give developers the tools to make apps for its mobile devices, plus a controlled environment in which to profit from them.

Fortunately, we don't have to wait for the Apple Watch and its app store to become available to get a glimpse of where things could be going. We have only to look at what pioneering companies in "wearables" are already working on, and then think about how their products will become a part of--or be absorbed by--the Apple Watch and Android Wear ecosystems.

"The vision of Sensoria is that the garment is the next ultra personal computer," says Sensoria CEO Davide Vigano. To illustrate his point, he rolls up his pants leg to show me a working prototype of the world's first smart sock.

I know, it sounds ridiculous. But as he explains with the help of an iPhone app that visualizes wireless signals sent by indiscernibly thin pressure sensors in the sock, it's good for runners who want to reduce their chance of injury. It is also potentially useful for monitoring the health of the elderly, since changes in gait are surprisingly predictive of other health issues. And yes, the smart sock is washable.

Sensoria also makes a smart bra and smart shirt, both of which can measure heart rate. And here's where things get really interesting: For all makers of wearables, which until recently have been dominated by the glorified pedometers known as fitness bands, fitness applications are just the beginning.

Using a few more of the same sensors it already carries, Sensoria's shirt could measure not just the frequency but the pattern of a wearer's heartbeat, Mr. Vigano tells me. Like the fingerprint sensor on new smartphones, the unique shape of the electrical signals generated by our hearts are a biometric. Add in a payment terminal not dissimilar from the ones that will roll out with Apple Pay, and the wearer of a Sensoria undergarment could soon find herself verifying payment for her next coffee via technology in her bra.

One company, Bionym, is already doing something like this. Their Nymi wristband is an ultra-secure means of personal identification. Put it on and touch it for four seconds, and it takes an EKG with a fidelity comparable to what you would get in a hospital. It is then matched with a previous distillation of your heartbeat pattern stored in the cloud. Once you put on a Nymi, until you take it off it uniquely identifies you. Current applications include unlocking your laptop or smartphone, but Bionym CEO Karl Martin tells me his company is also working on a version that can be used for contactless payments, just like many smartphones (including the iPhone 6) and the Apple Watch.

During Apple's presentation, Apple Vice President Kevin Lynch announced that BMW has developed an app for the Apple Watch that will allow users to lock and unlock their BMW i-series electric vehicles. And Mr. Martin told me that new bluetooth-enabled locks from companies like Lockitron and Kwikset mean that the moment you walk up to a door while sporting a recognized wearable, it can unlock without a touch.

Put all these possibilities together and what you get are a suite of functions that could almost, but not quite, be conveniently accomplished by a smartphone. Just as Uber could in theory work on a PC but didn't really make sense until the dawn of the smartphone, body-wide wireless networks and computers we never take off will create applications that simply don't exist yet. Wearables won't just appeal to fitness nuts and quantified-self geeks. They will appeal to everyone, because they will be the primary, perhaps even the sole way we identify ourselves to a world full of smart objects.

Established makers of wearables have seen this trend and are already getting in front of it. On the same day that Apple announced its watch, Jawbone, maker of the UP wristband, announced that they were opening up their quantified self-software so that anyone could connect it with devices other than the UP--including the Apple Watch.

Or in other words, amid the hype about the Apple Watch being a fitness device, or a timepiece, or a status symbol, it's the applications that Apple and others aren't even talking about, the ones that are thought up by countless developers jumping on board the platform, that will make us wonder how we ever got by without them.

"We're at the genesis of the sensing industry," says Mr. Vigano. When smartwatches are cheap enough to be ubiquitous, and if Sensoria becomes, as Mr. Vigano claims, "the Gore-Tex of wearables," and if the Jawbone UP and the Nymi wristband become features of all smartwatches, what then? These devices will be the way we connect ourselves--directly--to all the technology that surrounds us. And opting out simply won't be an option.

Follow Christopher Mims on Twitter @Mims or write to him at christopher.mims@wsj.com.

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