Watts are a measure of power put out by an amplifier, and information about how many is useful to get the right speakers to pair with the amp. In the case of Sonos speakers, this matching has already been done and this information is not relevant for that reason. What you need to know is how loud they can go, and knowing how many watts the amp inside delivers to the speakers inside, will not tell you that.

Other than for boasting purposes it’s utterly irrelevant, since you don’t know the sensitivities of every driver (they each have their own amplifier).