Recycling recipes adds a way to salvage some of the materials from the items you don't need anymore. For example, if you don't need rails anymore, you can recover an Iron Nugget instead of throwing the whole thing away. The addon also offers ways to craft items that could be previously acquired only through finding them in the world. It makes your life easier overall with such simple additions, like the ability to combine two slabs into a full block.
Make sure to enable Experimental gameplay features in Settings.