You probably thought that large ships had a stately, dignified birth off the docks and into the water. You’d be wrong. Like teaching a small child to swim, you just throw them over the side and hope for the best.
Reddit commentor DougyM does provide some explanation as to why this dramatic method is actually preferred:
You need to effectively have the entire ships weight distributed evenly along the hull, sliding it in sideways ensures that no one part is under more stress than another. Going forwards down a slope would focus the ships weight on either end of the hull and result in damage up to and including actually crushing the middle of the ship in on itself.
He fails to mention that the throw-the-huge-boat-over-the-edge method has the added benefit of being totally badass.