The idea here is to have favorable trades with the enemy laner, and be able to out CS them as a result, with the ultimate goal of getting/staying ahead in lane, and eventually becoming unstoppable force in both 1 on 1's and (more importantly) team fights, so your team can jump on your back to win the game.

If you 100% know you play more passive, or are in a matchup where you'll be forced to do so, you can go with Windspeaker's over Thunderlords for the buffs to your Q and W. Thunderlords just makes for really good "get in, do some damage, get out" kinds of trades, which Taric can struggle to do, especially early game. However, if you can land an E and follow up with 2 quick autos (Taric Passive) at level 1, you'll then get a thunderlords proc, and from there CSing should get a whole lot easier.