watchOS 3.1 focuses on bug fixes and performance improvements, following the launch of version 3 last month, which comes pre-installed on the new Apple Watch Series 2, and is also available as an upgrade for the original model.
The developer beta is - as its name suggests - intended only for developers to use for testing and evaluation purposes, and you should consult Apple's documentation before downloading and installing the new version onto your Watch, to ensure that you're aware of any significant issues in advance.
One particularly notable issue, for example, affects Apple Pay, but there is a workaround available:
A payment card may show as unavailable when attempting to use Apple Pay after restarting Apple Watch.
Workaround: Dismiss and double-tap the side button again to make the payment.