The table in Jungling Overview goes into greater detail.With the exception of boss Monsters, Monsters do not increase in level (stats) until after they have re-spawned.
Junglers rely on killing neutral monsters in the jungle to keep up with their laning teammates in terms of gold and experience.
In a standard 5-on-5 game of League of Legends, 4 players on a team will be Laners, and 1 player will be a designated Jungler.
And of course, the more farm a team accrues, the greater their chance of winning the match.
There are other important tasks that a jungler is able to perform, due to their roaming nature preventing enemy champions from spotting the jungler through the fog of war.
As their level goes up, so does their reward gold and exp along with their damage and defensive stats.