Suppose that's true enough. Generally speaking, though, I consider the jungler's job to maintain global pressure.
If your enemies are playing cautiously because they're afraid a gank might be coming, he's doing his job. If he's securing neutral objectives or counter-playing the enemy jungler, he's doing his job. If he's providing vision for your lane as he moves around the map, he's doing his job.
And yes, of course, if he's ganking lanes, he's doing his job as well.
But let's be honest - it doesn't always pay to gank lanes, and there's plenty of other ways to make your presence felt as a jungler.
I once played a game with a Nunu jungle - one of the best I've ever had - that never actually ganked. He would simply stop by lanes as he moved from one thing to the next, throw a snowball at someone, eat a minion, and run away. But he maintained vision everywhere, kept the enemy jungler from doing anything useful, secured every dragon, and kept their whole team terrified.