Vision is a very important asset in League of Legends. The more information you have about the enemy the better prepared you can be for an engagement. Conversely, the less information the enemy has about your team the least prepared they will be.
Ciderhelm’s tutorial covers how to ward and counter-ward, the use of Clairvoyance and provides numerous examples of Ward placement on Summoner’s Rift. No serious League of Legends player can afford to ignore this valuable resource.