The Dark One and the Creator exist outside of the pattern, but they do have influence. The DO's influence is related to the size of the bore in the Pattern that imprisons him. His touch is more direct, and he may well be slowly destroying the Age Lace; he is definitely affecting the normal cycles of the world. The Creator has a more indirect approach. He spins out the souls of his most powerful channelers, as well as those with important Talents that help the cause of good. Using T'averen is the best example. The Pattern is actually changed by the actions of an individual, and this helps achieve the goals the Creator wants. Ironically, chaotic change seems to be necessary for good to win. What you should realize about the DO's influence is that his ulitmate goal is the destruction of all the Creator made. All the Darkfriends, especially the Chosen, are being duped. Shai'tan won't remake the world if he wins, he will unmake it.
To answer why the Dark One exists at all, it is balance. The Universe needs balance, so there must be evil. One side can supress the other to an extent, but it all seems to balance out in the end. Comparing the age of legends to the current age is proof of that balance. No war, very little crime, and a Utopian society during AoL, changes to hell on earth in just a few years, and ends up a constant struggle for millenia afterwards. So, in the end, neither side ever wins outright, they each get their turn holding the greater influence.