Players initally build fp producing buildings so they can level their great buildings (normally via guild swap threads and sniping).
After awhile they get their Arc to level 80, which allows the player to participate in 1.9 swaps to build up their other great buildings, and to help others raise their great buildings by locking positions easily.
Leveling up the other great buildings opens up just about every other type of gameplay available, whether trading goods acquired whit a Chateau or pummeling troops with attack great buildings.
In all cases these things become possible because of forge points, and one of the best sources for forge points early on is the SoK.
The SoK becomes such a fixture that even after a player doesn't really need them anymore, giving up that reliable stream of daily fp isn't considered an option.
I am hoping that some day I will be able to get rid of all my SoKs and replace them with something more attractive and at least as powerful. A city based around Cherry Blossom buildings, with other attractive special buildings and my gb's interspersed is high on my list, but is also a long way down the road, if ever.