Homster

Main reason for header tank on your model (and on a real boat, where it is called a day tank) is to allow air in the fuel line, sucked up when the fuel was sloshing about, to have a place to "escape". The 'foaming fuel' caused by balance problems also has a chance to settle before moving on to the engine. 2nd reason is slightly increased capacity. 3rd is better visibility of fuel state (some main tanks are hidden pretty well). Last reason (and probably most common) is it's just damn COOL!

Elmo Te