Why double plank?

[Drzeus]

I'm not yet at the planking stage on my model but was wondering about HMS Victory's actual planking. Having seen a few of the conservation videos, it is clearly not double planked. Yet I see people frequently double plank their models. I understand why if your first layer is a bit naff. But otherwise, why do you do it?
(Definitely a noob question!)
Here's an interesting page on repairing Victory's planking (I'm impressed by the clamps!)

HMS Victory replanking - T. Nielsen & Company

HMS Victory replanking by T. Nielsen & Company in Portsmouth Historic Dockyard. HMS Victory is one of the most famous ships in the world...

tnielsen.co.uk

 

[Dave Stevens (Lumberyard)]

in kits hulls are double planked because the bulkheads are too far apart to produce a proper shaped hull. Planks tend to lay flat or create low spots between bulkheads because there is nothing behind them. On real ships framing is placed close together giving the planking a uniform shape to plank over. When the first layer of planking on a model is done wood filler is used over the hull and it is sanded smooth and even. A way to avoid the double planking is to fill the space between the bulkheads with filler pieces and shape the hull. Planking a hull does not hive the shape it is only a thin skin over a hull that has been shaped either by double planking or filler pieces/

 

[Dematosdg]

Also depends on the kit. Here is an example of The Endurance, by OcCre. You are force to apply a second planking due to the way the kit construction was developed. See short video and images to understand.