You farm because skills cost a ton of sp in later lvls. Some people farm at 16 in the bandit den due to the insane spawn rate of archers. Also, with zero gap leveling, you won't get enough sp you need for your skills.
Some people farm all or most of what they need at lvl 16 (sometimes up to 300k or even more). You can usually use all of the sp you have farmed as soon as you are finished.
Pro: You gain sp extremely quick, You are able to lvl faster from lvls 17 and up with a zero gap
Con: If you can't get your hands on a sox bow farming will be much longer, Incredibly tedious and monotonous and it can take a few months grinding a few hours a day doing the same exact thing for a few months (depending on the amount of sp your char needs.)
Some people lvl up keeping a certain gap between their mastery and their actual lvl (i.e. some people lvl up through the entire game alternating from 6 gaps to 8 gaps to 4 gaps or what ever). You usually use your sp to gain only the skills you
need while maintining your gap.
Pro: Less monotony, more variety (sort of

)
Con: Slower leveling, careful planning needed
Some people farm at specific lvls with a 9 gap. These lvls are usually lvls in which you can use an SOS weapon for each degree (16, 24, 32, 42 etc.) Some people also combine this with gapped leveling. You would use your sp the same way you would with gap leveling. If you don't want to farm everything at 16 and want to have plenty of sp, I suggest following this method.
Pro: Less monotony, feeling of actual progress, better drops and more money(at higher lvls)
Con: SLOW lvling, can be just as tedious as farming at 16
Keep in mind that I am a noob, but this is all I have learned in about 6 months of playing.