Originally Posted by Red Scorpion
Would someone be able to tell me firstly, why one is needed
It is common to fit a drain bung low on the hull so that any water which does find its way into the hull void can be removed. Some manufacturers are so confident about their water tight integrity that they don't bother - its not uncommon for owners of those boats to post here "I've got water under the deck and no way to get it out" !!!
In a perfect world that space would be sealed and dry - but any through deck fittings (cabling, fuel lines, seat screws, hatch covers etc.) run the risk of some leakage if not done with the utmost care. Some brands even intentionally drain anchor lockers etc into that void where a bilge pump can remove almost all the water.