They make it more likely to obtain materials from items, even materials that you wouldn't have assumed would be there. For example:

Before upgrading Woodworking, if I refined maple into sanded maple, I got sanded maple. If I deconstructed a bow, I might get more sanded maple, maybe a Pitch item to improve stuff if the bow happened to be a Green item.

After upgrading:

-I've gotten more wood mats back than usual after deconstructing.
-I've gotten improvement items such as Pitch from items more often, including occasionally getting them when I simply refine raw wood.
-I have a better chance of getting the gems from items I deconstruct/randomly from refining raw wood to give items traits.

If you're a serious crafter, get this perk. Do it now. It's very, very worthwhile, and means you'll be relying less on deconstructing high level items to get your tannins/resins/etc. It isn't a staggering amount the first time you upgrade, but it stacks and gets to be very noticeable indeed.

(Not to mention, it has the added benefit of sparing you from those people running around right now trying to sell crafting improvement items for ludicrous sums. Much cheaper to get it yourself.)