People have already hit all my reasons to use fascia board - hide wiring, place for labels, lip to keep locos from 4 foot plunge to floor, etc. - but I have one more. I build modules, and the fascia board helps protect the edges during transport. It's easier to touch up a little black paint on the end than have to re-sculpt scenery.
I would think you would want fascia boards for the same reason you have a body on your car... ...keeps mud off'n the innerds! :ru-tongue:
I paint the edges of my pink foam with a foam-safe spraypaint but I use the fascia to protect the front / top edge of the foam from getting knicked, crushed, etc. as Steam Donkey mentioned.
Being that I have never "finished" a proper layout......I am assuming the best time to install fascia is after the majority of the wiring, and under layout stuff is done correct?
I installed mine after all the trackwork was in and most of the feeders were dropped from the rails and soldered. My NCE throttle panels and holders are mounted dirrectly to the fascia so that is why I went ahead and installed.