It's mainly a gimmick to boost sales. But it so happens that CNC'd parts are often made of a better grade of aluminum and so they last longer - not necessarily because they've been CNC'd. Cranks are a good example. Echo Forged are made of 6061 grade aluminum. It's a poor grade at least from my experience, the tapers get damaged really quickly. Then you have Echo CNC cranks which are made of a much better grade of aluminum, are also forged and CNC milled to give them that special look. Most if not all 7075 cranks are CNC milled. So in this case, they're bound to last longer - not because of their finish but because of the material used. Such experience leads people to believe CNC parts are actually stronger. In some cases they are since machining parts from a solid block of aluminum means there is less chance of small air pockets or micro cracks in the material.