As far as I know, and what I've seen - with a front cut you get the front brakes, dash, gearbox, wiring loom, and engine. Lots of useful bits and pieces. If you're doing any sort of conversion (manul conversion, or engine into some other car) they are the sort of things you need. If you're just using it to replace the engine then you can chuck out the rest of the bits or sell them.
Lots of people buy them and just flog off the bits they don't need. Works out cheaper in the end by the time you've sold off all the "extras".
I think they're popular just because in Japan its easier to chop the car into two with a big chopper thing (technical name), than dick around pulling the engine out, gearbox out, separating everything, etc, etc. Then it can just go into a container as one piece, which makes it easier for shipping in many cases.
The panels are probably removed a) because they can get a good price somewhere, in japan or back to here as young people like to crash their skylines B) they'd probably just dented, scratched, etc anyhow when the cut is being chucked around for shipping.