I think the watch is analogous to Google Glass. Glass is, essentially, an accessory. It adds a remote display, camera, and audio functions to an android phone. (It has bluetooth, not a cellular radio)

I think the Apple Watch is similar.

But Glass's full display makes it capable of being a standalone device (once the guts shrink enough).

Before long, a Watch can also be self-contained, technically, but remains limited by form-factor: tiny display. Unlike Glass, it must remain accessory... or so it seems.

But consider: the iPhone is primarily a consumption device. In many ways, it is predicated on many use-cases not requiring a full computer. Is it possible a Watch-size device will similarly turn out to cover many important uses? Obviously, you can use it as a phone. For music/video. To read txt messages/short emails. Casual games.

It's a real consumption device (unless Siri gets unrealistically good). Maybe many web-site functions (i.e. the actual use of the website, not actual current website) can be delivered via Watch: lookup opening hours; store locators; product list/price/specials. Perhaps Watch versions, as we now have Mobile versions - and of course, Watch App versions).

After getting powerful enough, smartwatches could be full "peer" devices in your Personal Area Network, though.

Rather than tethering the watch to your phone and having the watch just being treated as another phone peripheral, you could just as well do the opposite: have the phone tether to the watch to serve as e.g. a keyboard and secondary display.