"Underscore" David Smith, developer of apps such as Pedometer++ and Sleep++, has been thinking about which kinds of apps make sense on the Apple Watch as it is today. He considers three major types:
I agree those three types make sense for apps on the Apple Watch as it is today. While I use Complications frequently, I haven't found myself using any from third-parties yet. So far I prefer the standard Weather, Timer, and Activity Complications.
However, Notifications and Sensors are both huge for me and why I love my Apple Watch so much.
Deliveries notifies me when that thing I ordered has reached my doorstep.
MacID let's me lock and unlock my Mac over Bluetooth with ease.
David's own Pedometer++ has been vital for improving my personal health since it ties into the Watch's accelerometer to help me see a better picture of my daily activity.
All of my favorite Apple Watch apps use either Notifications or Sensors, or some combination of both, in an extremely practical way.
Other apps are on the Watch just to be on the Watch, without having a sensible purpose. These are what causes the Watch to be perceived as having unclear purpose.