first thing, you don't know how to use the word: "literally"
second, ganking is the jungler's job amongst several others, you see, in game each role has a purpose, you don't go jungle because the monsters are pretty and you like smacking them, the whole "jungler" thing was something that became a thing back in season one when the norm was to have 2 players in top and bottom and 1 player in mid, eventually people realized that killing the monsters on the jungle gave close experience to the one you won in lanes and there were a few champions able to withstand the monsters punishment from level one (
) so the role started to become the norm because it optimized the resources on the map.
there was but a small detail that people weren't happy about and it was the small difference in experience that a Jungler had with a "laner", even in bot lane where 2 people lingered all the time, the jungler was staying behind in experience, the solution for this became what we know today as ganking and taxing, the experience the jungler was missing on the jungle he got it from either killing the enemy or obtaining few bits of experience from minions, either last hitting them or simply staying around until jungle monsters respawned.
Ganking changed lane dynamics since staying behind the imaginary line that divides summoner's rift in half was more important now than ever, considering also that there were some champs that were starting to be used as junglers who were able to gank at lvl 1 (
)
Enter supports who were tasked with the vision game since wards were pretty cheap back then and they had the most free time out of every role. Supports playing up the vision game made Junglers understand that there were some finesse to ganking so they relied on their teammates to ward around in order to track the enemy jungler so that they could either counter gank or punish the enemy jungler by showing up on another lane or what is known today as lane pressure.
Enter 
who were specialized counter junglers who didn't have much of lane pressure but who's pressure was aimed at the enemy jungle itself, counter jungling became a thing then. So we have a bunch of terms being thrown around and a bunch of different junglers and changes to the monster camps and whatnot but the most important thing is that the role of a Jungler started to be outlined barely during season 3.
People (and riot) understood that the Junglers were a thing and they were here to stay, so work was poured onto the role itself and the bunch of changes that we know of started to come through the door until a definition for the role was able to be coined.
The reality of the jungler is that it exists because it's the best way of optimizing the resources of the map BUT ALSO AND MORE IMPORTANTLY each team has a jungler because each team has a jungler... exactly that, you play in the jungle because the enemy team has someone playing in the jungle and your grade of effectiveness is mostly dictated not by what YOU do but by what the enemy jungler does and how you react.
the most important job a Jungler has is to stop the enemy jungler from outscaling him, simple as that, if the enemy jungler ganked and took a kill then he's above in experience and gold, what will you do? farm jungle and hope he miraculously stops scaling? you HAVE to catch up and farming jungle alone won't do the job. You have many options like making another enemy loose resources somehow, not necessarily killing them but making it so they are more wary of making plays.
One last thing, a jungler has to have presence, if the enemy team figures out that you're "that" kind of jungler who stays in the jungle until level 6, they KNOW, they have at least until level 8 to kill the enemy laner and they will get that kill because they can fully commit without a care in the world for you to show up, they could have a more cooperative jungler and turn the lane into a 2v1 and where will you be?
