Apply it to real world and you'll immediately see the benefits of map visibility. How well would we be equipped for war without radars? How valuable is stealth warfare etc?
Being able to sneak up on opponents is too strong of advantage to let any team have for free. Wards allow you to prepare for incoming ganks, to move quickly on map objectives, and even for some champions provides as a way for escapes.
The most obvious understanding of this comes to bottom lane. As a support, your job is to let you carry farm as much as humanly possible without being interrupted. Wards allow you to watch for incoming jungle ganks, roamers, etc.
The other role whose main job, among clearing neutral minion camps, is warding, would be the Jungler. As the bottom lane support is, quite frankly, stuck at bot lane, the roaming Jungler has a better chance of passing through areas that could use wards that would make or break a game. Until lane phase is over and the support starts to move around, it is important for the Jungler to ward the rivers to prevent ganks on top and mid, attempts on dragon and baron, counter jungling attempts by the enemy, as well as to provide constant vision of the other team so that your own knows when it is safe to pursue certain objectives.
For example, a ward in their jungle near the Blue Buff would show you that the enemy Jungler and/or Mid is currently not killing it and it is somewhat safe to steal it. At the same time, if the ward shows you both their Jungler and AP Carry are at the Blue Buff, it is safe to do a 3/4/5vs2/1 team fight on bottom since you know they will be receiving no reinforcements.
Seeing the map just allows you to do/access/defend/attack safer, smarter, more efficiently.
in the ggClassic Epik vs CLG game, CLG ran a "kite" comp.
They basically ran fast move speed champs that had very good poke and would kite epik until they were low enough to collapse on. It worked so well it was scary.
I have also seen true damage comps, with vayne, olaf, cho. It is kind of one trick but the focus target is guaranteed to die.
For a college tournament, I had a few comps pre-planned based on certain team synergy tricks, not really high elo or anything more theorycraft on beastly mechanics. We ended up using a couple:
Super stun gank heavy - Sion, taric, ashe, veigar, udyr
suppress heavy - malz, ww, scarner, taric, trist
Best Answer
They can still see everything you say in chat, and all of your minimap pings unless they ignore you as well. You will no longer be able to see their minimap pings.
The commands to ignore someone for a single match is: (entered into chat)
To ignore someone permanently:
Sources: http://leagueoflegends.wikia.com/wiki/V1.0.0.86 under 'General' for minimap pings. http://na.leagueoflegends.com/learn/chat_commands for ignore commands.