Says it works on the Apple Watch but only refers you to use your phone

I was looking for an app that I can keep track of my water intake on my watch. The app works on the watch but only to refer you back to use your phone. Whats the use of offering the option. I dont always have my phone handy at work so it makes it harder to track.

Migyz about My Water: Daily Drink Tracker, v2.2.2