Pavan Rajam, writing about a potential iWatch and its place in your device ecosystem:

The smartphone is easily the most personal device we own today. It is our life in our pockets. It is our connection to the rest of the world. We take them with us and use them where ever we can.

Yet there is still a gap, there is still some friction. There are moments when we are without our phones. We can lose them. They can be stolen. On the less sinister side, we give them to other people to look at something, or to hold for us.

There is simply no guarantee that the person using a smartphone is the person to whom it belongs. That’s problematic when you look at use cases which benefit from stronger ties between a user’s identity and presence in the physical world. Use cases like health tracking, home automation, payments, and transitioning between devices. These use cases benefit from something better, something more omnipresent to the user than a smartphone.

This is an excellent point. A wearable is personal. While someone else might use your phone, there are far fewer cases where someone would use your wearable, which makes it the perfect sensor for gathering truly personal data (e.g., pulse, stride, glucose/insulin levels).

While a watch is a wearable, it is not the perfect wearable for all use cases. By its very nature, a watch’s shape is constrained to be of a shape, size, and weight to fit unobtrusively on your wrist. Google Glass is a wearable that serve a very different function, so its place is on your face.

One advantage of a watch as a wearable is that it occupies some very special real estate on your body. If you want to communicate, your wrist can quickly be extended as needed. For example, if your watch was part of a payment system, you can extend your wrist to bring it into contact with a payment terminal, much more easily than digging out your phone or glasses.