I don't have an apple watch, but I have bluetooth earbuds that will display in the battery widget.
it's important to note that the battery widget does not show up unless you have an accessory that works with the bluetooth battery info. Also, not all bluetooth headphones (older, cheaper, etc) don't have this feature. So, if you only have the iPhone and nothing with bluetooth, you will not see the battery widget.
as it'd be super pointless.
it kinda still is, when you only have your AppleWatch and your phone.
The main appeal is for headphones or speakers that do not have a battery indicator by themselves (apart from maybe an LED light that goes red at 10% or so)