I travel every week, and I am a AW owner since it launch.
As the other says, it's just an extension of your phone. Is it usefull will depends on how you use it (both your phone and watch). Even though no data plan, offline map (NOT the native map app) still works. I always leave my phone in airplane mode at outport to avoid unnecessary calls and possible data roaming, just manually turn on BT and Wifi after switch to airplane mode to enjoy using my watch and offline map. In this config, I can even accept Wi-Fi call (for free) when I am under Wi-Fi network.
One of the handy function the watch is the ability to display multiple time (e.g. I can easily setup a face to display my home time, current time, and UTC). And it will automatically update the primary time to your current location time zone.
The haptic engine makes you can sleep on train or bus without worrying missing station. You can now properly store your phone in a inner pocket to avoid the thief. And set an alarm to wake you up by vibration only, so you won't make any noise to others (e.g. you have to get off the train at mid-night), no need to pull the phone out when the alarm ring (which may be a bit hard if properly stored to avoid thief during sleep), and you won't miss the alarm no matter how noisy out there (or with ear plug on).
And of course, you can put all your boarding pass (or even hotel key) on the watch if the airline (hotel) support it.
The 2m long lightning cable usual works well in hotel. Most of the hotel have power socket just next to the bed. When you charging the watch (during sleep), you can leave the watch next to you in night stand mode (on the bed). And all you need to do now is pat your bed, then the watch face will come on and show you the time (and if there is any notification), no need to wake up.
What offline maps app are you referring to that works on Apple Watch? Got a link?