As Tekkub says: StealYourCarbon is a simple reagent restocker addon. When you visit a vendor who sells an item you have told SYC to buy, it buys it for you. Never run out of water again!
And that's exactly it. On my druid i just told SYC to restock reagents for buffing as well as two stacks of something to drink for Mana. My Death Knight is happy with dust always availble and Rogue with his poisonsnever missing from his invetory. But be warned, SYC will not help you to remember to go to vendor and open trade :)