Based on the description on their site, the controller includes a built-in battery: "8.39 Wh Li-ion battery, 35+ hours of gameplay... "
That was disappointing for me. Specially condidering the Steam Frame's controllers make use of AA batteries: "One replaceable AA battery per controller, 40hr battery life"
AA Batteries might not be as convenient to use, but being able to replace them is a great advantage. All my Xbox360 controllers still work fine, but none of my PS3' Dualshock 3s.
The official docking station could be used to recharge (rechargables) AA batteries so the functionality could remain the same.
Or they could have included them with the controller at-cost instead of making us pay $15 to $30 (the official battery packs are $30) PER CONTROLLER - many of us have more than one.
Considering the price difference between an Xbox Series controller ($40 on frequent sales) and a DualSense (rarely below $70), maybe they did.