levelling makes you more proficient at manufacturing a setup for your character with skills and gear. when you feel your character progression is strong enough, usually people try to compete in player-versus-player scenarios competing for castles, fortresses, or hunting zones for higher levels.
at a certain level of the game you may compete for the Olympiad and possibly one of the 30-odd Hero spots (a hero is the best player of his class). heros have special skills, special weapons, and access to special hunting zones.
what do u get from leveling?
is there a main quest u start at level 10 or is it higher?
Leveling is a means of gaining power. The point of the game is to capture Castles. To do so means you must be stronger than your opponent. Leveling is simply a way to do that along with gear and politics.
Also, at level 20 (after doing a quest) you can use better gear, magic gear of D grade. At lvl 40 you use C grade, 52 B grade, 61 A grade and 76 S grade. The power/magic of each higher grade of gear is significantly more.
You will find that much of your game effort goes toward affording and acquiring these different gear sets.