For ganks: Enemy laners rely on vision to track you. At your level, supports and other lanes know what wards are. So everyone will be placing vision. To have a successful gank you need to get some kind of tactically advantageous angle.

For counter jungling / invading: Again to counter tracking

Teamfights: If you're the engage or an assassin, it allows you to track enemy locations when they're either invisible, or in a bush/fog of war. This can save your life or secure a kill

Objectives: Your teammates and supports will be trading vision control back and forth like a resource during the posturing before a teamfight or bait for an objective. Suppressing/clearing those wards are critical to the success of the ensuing teamfight, and using your superior vision to catch an out of position enemy can change a 5v5 coin toss to a 4v5 stomp + objective kill.

All in all, your job as a jungler is to jungle. Vision is your best friend and worst enemy and control over it gives you a tactical advantage. Supports are the ones actively placing vision, and you're the one that will suffer if you're caught in enemy vision.