The answer to the first is exactly what the name implies. It is a needle valve that you can adjust in flight to control the high speed mixture. This is helpful for a lot of people who want to lean the engine just a little on take off for a little extra power, then richen back up once they climb out. Or, if you take off and find the engine is a little rich, or lean, you can correct it from the transmitter. Yes, it's quite a benefit if you have the room for the extra servo and an unused proportional channel on your TX.

John