There are MANY different types of coatings. In fact, the D675 has the piston skirts and ring lands coated with a VERY low drag coating called Diamond-Like Carbon (DLC). I have used DLC and it works VERY well - reducing friction by more than half.
Low friction coatings reduce friction is high load areas like piston skirts, some types of bearings, cams, etc. This frees up power at higher rpm.
Oil shedding coatings on the crank, rods, sump, etc. and a crank scraper, reduce oil build up on these parts and reduce windage - the added weight of oil, and its tendancy to foam when beaten by the rotating parts. These coating work best in high revving engines like cycles.
Reflective coatings on piston tops, chambers, and exhaust ports are used to improve thermal efficiency. They keep the heat in the chamber (not lost to the cooling system), which makes more power. They can also be used on the valves to prevent the intake valve from getting hot and transfering this heat to the incoming air/fuel charge. They also reduce the tendancy for hot spots in the chamber (and exhaust valve) to form and reduce the chance of pre-ignition and detonation. This is especially true at lower rpm when heat in the chamber has more time to soak into engine parts.
Depending on the engine design, the coatings used, and the rpm the engine sees, I have seen improvements from 3-5% in peak power, and even more lower rpm torque improvements when the engine is modified and tuned to take advantage of these coatings.
Lower friction, better efficiency, improved throttle response ALL can be had with coatings. Many OEM's are starting to take advantage of this technology, and racers have been using it for decades.
Also, cryo-treating the pistons can greatly increase the lifespan of the skirts and ring lands - keeping the bore seal better for up to 50% longer.