What are these fence builders doing? Painting the ends of posts? Good guess, but that isn't paint. That is a 2 part epoxy that is entirely waterproof. It is also much harder than wood once it cures, so if you seal the post above ground level a little it will prevent damage from weed trimmers and the like.
Sealing the end grain with epoxy is like plugging up the end of a straw and will help prevent the wood from absorbing moisture from the concrete footing.
Rot typically forms within 6" of the surface, where the ground has plenty of moisture and oxygen to start the wood rotting. If you can make the post non-porous, the moisture cannot enter.
What does it cost? A couple of hours work and roughly a couple hundred for materials to do an average yard.
How long will it increase durability? I would expect a decade or more--however we have only been offering this option for a few years...so ask me again in 40 or 50 years.