The keel is is the backbone of the boat, it is the only, or at least the oldest, man built structure made to resist three dimensional forces.Even if somebody only consider it as a "beam", when it is connected to the stempost and to the prow it becomes the center of the hull, able to resist the joint force from waves, from the load, from the oars' or sails' (or engine's) push, and its own weight.
From the very moment these parts join up, they will not be anymore just "parts", but will eventually be a boat.
Now the keel must be accurately fastened to the cradle, plumbed to the slope and level, in order to put in place any remainig part with required accuracy, to be sure of the boat's simmetry.
