TubeNet

You can use it to play the petals more in tune (I think the Arban book has a 5 valve BBb fingering chart) and now you can really play the petal C flat with out a ridiculous amount of slide pulling.

Doug Black

Here is some information that you may find helpful:

http://galvanizedjazz.com/tuba.html

You can use the 5th valve in combination with others to play certain notes in tune better than you could with only 4 valves, and you can play chromatically all the way down to the pedal tone. As for remembering the fingerings, good luck.