I find it fascinating that the golf industry is like my one industry in that companies are innovating and inventing new tech with the primary (often sole) purpose being to enhance perception rather than reality.
Had myself a bit of a milestone yesterday.
Normally when I play casual at my home courses, I'll play from the gold (seniors) tees because they are well over 6,000 yards anyway. With the club championship less than a month off, I've moved back to the whites in an effort to prepare.
I had managed scores in the 80s a handful of times from gold on Kittyhawk's Eagle, but for the first time, I broke 90 from the white tees. It's 6,713 yards, 71.9/125. Posting it on ghin, it's easily the best differential I've had this year.
The "milling" you see on clubfaces are simply tool marks left by the cutter head. I think people started associating them with quality or performance (think Scotty Cameron putters) so companies stopped polishing their clubs smooth. It's kind of funny because it saves them a step in the manufacturing process, as tool markings are generally considered unfinished.
As I understand it, they don't make a significant contribution to spin. Grooves affect spin because the edges grab the ball (less so now than with the old box grooves) and the void allows water and debris somewhere to go so you have cleaner face to ball contact, much like the treads on a tire.