I'm using the right levers with BB7. My bicycle is equipped with SRAM Apex group.

The advice about pre-tensioning the cable comes from the simple fact that moving the actuator arm on BB7 makes the pads to spread apart at first (!). They move away from the rotor for the very small fraction of the initial movement of the actuator arm. And only after that, as one continues to move the arm, the pads begin to close and grab the disk. This behavior is easily observable from the outside. When I saw it for the very first time, I though that my BB7 might be defective, but further investigation and testing of several brand new BB7 packages revealed exactly the same behavior (not even mentioning that it has been reported by other users before).

I believe it is caused by some amount of lateral play that is always present in the actuator mechanism. The "screw" of the actuator can wobble from side to side in the caliper. When one begins to pull the cable, the first thing that happens is that the actuator screw tips up, under the force applied by the cable to the actuator arm. And only after that the actuator arm begins to move and the "screw" begins to rotate, closing the pads around the disc. That initial tipping motion changes the alignment of brake pads (and is often seen from outside as brake pads "spreading apart"). This initial realigning effect is exactly what we are trying to dial out be pre-tensioning the actuator arm. Note, that we are not really trying to move the arm per se. What we are trying to do is to dial out the lateral play of the actuator mechanism. Of course, in practice it normally means that we move the actuator arm up a very tiny bit before tightening the pinch bolt.

Pre-tensioning the actuator arm is not absolutely necessary. I used my brakes without pre-tensioning the arm for a while and they worked fine. Again, they will work fine without any pre-tensioning, if aligned properly. However, when the arm is ever-so-slightly pre-tensioned, just to dial out that extremely minimal initial portion of its movement range that makes the pads to spread/realigh, the brake feel improves noticeably. It also becomes much easier to adjust the pads for that ideal position where they don't drag and at the same time respond instantly, with minimal "dead zone" on the brake levers.