NGUI each UI has camera?

Sorry if this is a stupid question, but im a bit confused as to why NGUI has its own camera for each UI that it has.

In my scene I have a first person controller with the main camera, so are the other cameras just used to see your UI as your are developing it or is there some magic I am unaware of which uses the cameras when rendering the UI?

camera lets you use GPU to draw anything. using separated camera, combined single mesh and simple material for all GUI will draw in one drawcall, that is very-very cheap for GUI. so the best way to draw GUI is exactly camera and good-combined mesh, what NGUI do.