Do you remember, back in the day, when a bike was built out of steel in a jig? For those of you who don't, a frame jig helps the builder put together the frame in such a precise manner as to keep it properly aligned from start to end. That means the finished product is straight and true.
A frame built in a jig is going to coast down the road with less effort, much the way your car, with a proper alignment gives you greater control and less wear and tear on the tires. This results in less power being wasted and more going into propelling you forward. Also, a properly aligned frame has greater stability during a fast decent, a tight corner in a crit, and makes a steep climb a little easier.
How does this apply to today's carbon fiber frames? Many of the current carbon frames are built as monobloc frames, the reason is to visually clean up the lines. A frame that comes out of a mold will always be a little out of alignment. You may not notice it, that is, until you try a lugged carbon frame that has been built in a jig from start to finish. You will notice a level of stability that was not previously felt with a monobloc frame built in a mold.
Do a little research yourself. You will be surprised at what you learn, I sure was.