We discussed this in another thread. Some people just naturally understand how to manipulate the AI and how to get the most out of their turns. They still had to learn the game and everything, but nevertheless, they "get it."
Then there are players like me, who don't "get it." I played this game on-and-off for over a year before I began to understand what the game is all about. (And some would say I still have no idea what the game is all about.

) I have a difficult time "thinking in turns." And I have a hard time making calculations which I believe would be helpful in mastering the game. At the beginning of the game, for instance, it is difficult for me to compute how many turns it will take to build two warriors, a granary, a worker, and two settlers, and then compare that with another build strategy. Unless I spend 30 minutes with a spreadsheet, it ain't happening.
These things would have never even occurred to me had I never learned about settler factories.
Like Abegweit commented, it is not necessary at all to use settler factories to win on the highest levels. Many players beat Deity level before the concept was "officially" introduced to the CFC community.
But for someone like me (who has a difficult time with Emperor), the concept of settler factories helped quite a bit. It slowed me down, got me micro-managing, got me using my workers in a more thoughtful manner, and got me understanding how important just one turn can be.
Not that I attribute everything to settler factories, but the idea was quite helpful.