WHY COAT A VALVE?
The purpose behind coating a valve is to extend the part life and reduce friction. You're dealing with an intake valve and an exhaust valve. We'll look at the intake valve first.
The intake valve seals the combustion chamber on the intake port side of the head. Prior to that, it is opening to allow air and fuel to enter the combustion chamber. Intake valves usually do not suffer as severely as exhaust valves which see combustion chamber temperatures. Therefore, the primary concern is lubricating the valve stem and seat. We do this by the application of a dry film lubricant. This reduces the friction particularly in engines where oil flow is restricted to the head. The dry film takes over the bulk of the lubrication chore. It is still advisable to coat the face of the valve in the combustion chamber to help retain combustion chamber heat in the chamber. This also reduces the operating temperature of the valve. It reduces the temperature of the back of the valve so the incoming air fuel mix does not pick up as much heat from the valve, as it would, if it were uncoated. Intake valves also suffer from reversion. This means that when the intake valve just opens, combustion gases back up into the intake port. This is reversion. This causes small particles of carbon to collect on the back of the intake valve. Reversion is the main reason intake valves accumulate so much carbon. DFL are very slick and the carbon will not stick.
On the exhaust side, we have a more severe environment because the valve is seeing combustion chamber temperatures when it opens. Which on a normally aspirated engine, it can easily be at 1350F; on certain engines this can run even higher. So we definitely coat the face of the valve with a thermal barrier to reduce the heat that the valve absorbs. Then coat the back of the stem with a dry film lubricant. Again, it's very critical on the exhaust valve stem because the heat can reduce the ability of your oil to lubricate. Consequently, the permanently bonded, high pressure, high temperature lubricants work extremely well at reducing friction and wear on the valve and the guide. Valve guide wear at the port side is due to carbon building up on the stem, as the valve open and closes this carbon acts as sand paper. Carbon does not stick to DFL well. This is an added benefit of DFLs,
In some cases, it is advisable to coat the back of the exhaust valve with a thermal barrier. Titanium is an unusual metal in that titanium only likes titanium. Titanium corrodes and erodes badly when in contact with unlike metals. By coating the back on both the intake and exhaust valve after all machine work is done, you permanently bond the lubricant to the areas that will contact the valve seat thus reducing wear in this area and creating a better long term seal.