Why do panel lines? Well ,I'll tell you. (cue Monty Python music)
Having spent 4 years on a carrier, I got the chance to work around and on some of the greatest planes the world has ever seen. After the awe and wonder of actually running my hand accross an actual F-14, I started to realize something... those birds were filthy! After sitting around for hours with mechanics working on them, they'd be launched through a steam catapult and fly around in the dirty air for hours at a time. To make a long story short...
The paint on an aircraft is hardly ever uniform. After panels have been removed for maintenance and re-fitted, they always paint around the edges of the panels. Over time, paint builds up in all the little nooks and crannies of the planes, giving them darker lines around all of the panels. Most modelers don't worry about that detail, but in reality, the only time that you won't see a plane with panel lines is after a complete overhaul and repainting, or if the bird is the cag's and is used for PR functions, such as air shows (think Blue Angels.)
demono69
by the way, for all those wondering, the "69" is actually from my old ship, the USS Dwight D. Eisenhower, CVN-69. It is not intended to sound otherwise...