The why is simple - the rx and servo needs more than the 3.7v (nominal) voltage of a single cell lipo (1S). Booster steps the voltage up so rx and servos have full 6v to operate on.

As far as which is better, a booster or an rx batt, you'll need someone thats actually running 1/12th to comment.

This discussion might help some to -

http://www.rctech.net/forum/electric...e-booster.html