The large rake on a catamaran is a function of the relationship betweeen the speed of the catamaran, the drag on the hulls, and maintaining a workable center of effort on the mainsail.

In short, the cat moves at such high speeds, the hydrodynamic drag on the forward moving hulls greatly encourages the center of effort forces on the mainsail to move well forward of the front beam, (where the mast foot is located; as the main is moving through air, it has significantly less drag than the water-steeped hulls when moving forward). Crew and skipper will counter this effect to a degree, by moving back on the boat, but can only balance the forward force/effort only to a defined point (the back of the boat). As the center of effort moves forward of the mast foot (front beam) the boat hulls begins to go downward (trip over itself) into the water. The "stuffing downward" of the hulls can result in the "pitchpole", wherein the hulls submarine into the water, (and cause a nice boat cartwheel-google "catamaran pichpole". It is the most spectacular of sailing capsizing!).

To counter this effect and to generate greater boat speed, catamaran masts are raked backwards, allowing the hulls to pop up at speed, and on some, plane across the water. Thus, mast rake results in both greater boat speed and better control over the boat at high speeds. The effect of a raked mast is best visualized, not by looking at a catamaran, but at a windsurfer at planing (high) speeds. Those folks lean back, pulling the sail back with them and scooting the mast foot forward relative to the main. They begin to plane their boards, and go much faster.

As might be expected, mast rake is a dynamic control, and is commonly adjusted by the the teams to compensate for different wind conditions.