First, Apple Watch jailbreak will give you the ability to install third party apps from alternative app stores like Cydia. Cydia has very nice but risky apps which you will love but could not make way into Apple Store due to Apple strict restrictions.
Secondly, Apple Watch jailbreak will allow you access self-tune your Apple Watch for your own need but comes with some bad sides. For instance, you can turn off background processes to significantly extend the life of the battery of the Apple Watch.
Thirdly, once you jailbreak your device, it opens up wide possibilities for changing the Apple Watch's interface. For instance you can choose to run your watch with a completely different style of interface and maybe change the dock to complete the modification of the switches,system icons and screens.
NB: Jailbreak, voids your warranty of your Apple Watch, proceed at your own risk