Most apps don't need to be on the watch
For the most part, apps operate in two modes: engaging or "idle with notifications". A game is engaging, a todo list app is unused 95% of the time but occasionally sends useful push notifications. The most striking example of this is Dark Skies, an app that I almost never open but consider one of my primary tools because of its perfectly timed push notifications.
There are some apps, however, that request frequent and small amounts of attention from the user, like Citymapper or Uber. With Uber, you engage your phone and request a car, but subsequently look at your screen only briefly to see when the car is arriving. For people like myself that have quickly locking screens and a large phone with tiny pockets, this is laborious.
If you commute by bus, train, bike or foot, you'll want one
Citymapper is one of the best commuting tools I've ever used, but after the initial setup of where you are going, the remaining user experience is simply the quick glance to see your position on the map and information regarding your stops. You would never want to setup a route on the watch, but checking status updates on the phone is, like the Uber app, time-consuming and cumbersome.
As a bus commuter/biker in Los Angeles, I found myself constantly locking and unlocking my screen to check bus arrival times and estimated time to destination. I even bought a bike mount for the iPhone 5 so I could watch the Google Map while I biked and know where to turn. With the watch, apps now can push not only visual cues for directions, but also tactile cues -like vibrations- to indicate whether I should turn left or right at the upcoming intersection. The phone is used to setup the commute, then stays in my pocket during transit, freeing my hands to read, work or enjoy the experience with as little interaction as possible.
In this particular context, the watch achieves its goal: to consume the least amount of time necessary for the user to garner new information. The watch's limited space and restraints on user interaction, ironically, will make poorly designed apps increase the acquisition time for new information, so developers will need to be thoughtful and resistant to the "let's build a Watch app just to get PR" move.
Apple Pay: the sleeping giant
So does this mean that the Apple Watch is useless to anyone who doesn't use transit apps? Not necessarily. If Apple can push Apple Pay adoption to 98 - 99%, I believe it could bring with it an avalanche of watch purchases. Apple Pay has already reduced transaction friction and increased security for most users; having the ability to transact with even greater ease in the physical world (buying tickets at concerts, farmer's markets, grocery stores) maybe enough to capture the attention of Apple's entire demo graph.