If you don't ward it leads to very exploitable positions, essentially wards save lives and let you know what your enemy is doing. Junglers like Lee Sin, occasionally Vi, and Nunu can benefit from sightstone, Lee Sin is the biggest example, it allows him to track the opposition as well as make big plays.
This is a recent game of mine, I played mid, we got behind in kills, but that was due to poor team fight positioning, note the ward total.
That is the enemy teams ward total, it allowed plays to be made, like a roam down bot lane for me to get a double kill, So moral of the story, wards save lives