Warding is the act of placing Sentry Wards at target locations in order to gain vision of said location. To know where the enemies are at any given moment is a very important matter throughout all stages of the game. Wards provide the additional map awareness to get that information constantly. Where is the jungler trying to gank? Where are the missing enemies heading to? Having or not having information about the whereabouts of your enemies makes a huge difference. It allows you to tell when to go back, when overextending isn't that risky, when you are getting counter-jungled, when you can gank, if aeon/daggoth are still there to name a few. Without wards, you would just play a guessing game which is not only inaccurate most of the time but also very risky for your sole goal in this game: Winning.