If I remember the original correctly, the Cylons were the name of the alien race that made the Cylon cyborg toaster grunts, and possessed two brains (or perhaps it was three hemispheres, I think). That was the reason for the Battlestars in the original; defense of the colonies.
Less plausible for the new edition; the Cylons were created by man to make things easier. Some carriers and fighters would have made sense in the first war against the machines, but a Battlestar is an immense ship, and you would think that the money, personnel, material and such would be astounding, thus the reason there was only 'one' per colony. Of course, by the second war, there was a very justifiable reason to have such a large navy; the Cylon machines were still a menace, and the colonies were, more or less, in a cold war. But the fighters and ships before the first war does seem a stretch. One or two would have been more than sufficient in case of a civil war or a colonial insurrection.