You need the lines full of the mineral oil (fluid) for the brakes to work so he leaves the fluid in. Hydraulics works by you transferring force by the liquid, so when you press your handle it sends pressure through the fluid in the line that then pushes the brake pads together. If you take the fluid out and just have air, you will press the brake handle and nothing will happen. If you have insufficient levels of fluid, you will press the brake handle and you will get some braking but very little.
