Also, what were the driving forces behind giving buildings a coat of paint? I would presume the primary reason is for protection from the elements, but were there other factors at play? What were early "building paints" made of, and what type of materials were typically used (brushes, rollers, etc)?
In television/movies I always see "old west" towns consisting of raw wooden buildings--is this accurate? Was paint not available at this time, or did the cost/benefit (in both money and labor) just make it impractical?
Driving force part of your question is pretty easy: wood is degraded by light ( mostly ultraviolet), eaten by fungi, and insects, and heat plus water/humidity enable the rot and the bugs. In places of extreme cold or extreme dryness wood will survive a fairly long time ( so the very dry US Western desert is not going to lose those ghost towns very soon) . Paints can keep the wood from getting wet as easily, both by physically blocking the water and also by preventing the weathering by light, which makes the surface more porus, open to saturation. Paint also can deter insects. A white-washed log cabin will last longer than one left unfinished .
I am not sure how you could date when people discovered that some lime or plaster painted onto the lintel of a doorway kept the lintel intact much longer. Certainly the Minoan civilization circa 2,000BC was doing it. But it was doubtless discovered earlier: there are any number of earth pigments ( like the ochres) that can be essentially just dug out of the ground, mixed with a drying oil like linseed or mixed with some milk, then wiped onto wood every year or so to provide protection, and a rag or scrap of sheepskin would work for applying that.
Perhaps looking into the treatment of water craft (against leaks and rot) might shine some light on this question.
Well, wood is susceptible to a variety of things that can ruin its structural integrity.
Water can grow mold and lead to rot. Simply getting wet and drying again can expand and contract fibers eventually wearing them out.
Paint helps negate those issues. It keeps the wood dry. And fun fact; ever wonder why barns are red? Well rust is red(ish) and rust has some potent anti-fungal properties. Slap some paint with rust in it on some wood and you can bet it won't rot. Do a good job painting, and it will be sealed well enough that the wood won't get wet. Those two thing alone can extend the useful life of a wall or floor by a good decade if done well.
As to when? Fuck if I know. Humans have been painting shit since before we started writing. It's some prehistory stuff right there man. I know my written history, but I'm no archaeologist.