Custom rods are for people who know what they're looking for in a rod, mainly because they have noticed some important qualities that are missing in their off the shelf rods. Factory rods are built for the masses, the components used are not of the best quality, and they are not built with the care that a custom rod is built with. Having the ability to change guides and guide spacing makes a huge factor on the action of the blank and weight of the total build. Then theres the fact that certain blanks can only be bought as blanks, so in order to fish one you'd have to get it built. Theres also reel seat position, handle length, balancing the rod with a specific reel. Many, many, many factors. You build the rod to your specifications and end up with something that is truly unique just for yourself. You mentioned that theyre quite expensive, which is true, but they will likely last you a life time and since a rod that was built just for you you would never need to upgrade, plus they keep theryre value a lot better then factory rods when it comes to selling. In my eyes they actually end up being cheaper, epecially if you tie your own. For example a 13' raven im8 with a reelseat goes for about $250, i can build up that same blank with MUCH better components for the same price if not less.
Dimitri