Has its origins in gliders. Originally the radios we used (back in the 1970s) were aircraft ones, and the best ones at the lowest price were 3-channel units used on gliders. The third channel was used for releasing tow hooks, flight trim, etc. For cars they've been used for the things named above, because it was there!
Third channel is not used for PTs, as they are not controlled by the Tx. We use that slot in the receiver just to save having a Y harness for the BEC/booster/receiver pack and the PT together when running 1S. If you're running 2S, then you don't need the extra slot in the receiver either. HTH
