I highly recomend one becuase you can keep flying untill the main tank is 100% out of fuel... VERY nice fuel gauge.... that and it gives you 2 ounces of fuel.
Another reason it's there is that in extreme 3D flight there is a chance that the clunk in the main tank may get some air while the fuel is swashing around... The header tank makes this impossable because it is ALWAYS full of fuel... until the main goes empty.
LOL, what I do is fly up high till the main tank is gone then do hover manuvers till the header goes and auto down it the other 5 feet.