Basically, I'll give an example. Each class should have their strengths and weaknesses, a warrior should have heavy armor, because a mage wouldn't be strong enough, and a ranger couldn't be quick with it. 

the job change at 10 is not necessarily required, nor is it a game changer. 

Say you started as a ninja, 

Class: Ninja
Passive: Assassins touch - High critical chance
Passive: Acrobat - Lighter body able to climb and jump quicker

Skill: Shuriken strike - Spray shurikens dealing medium damage, High chance of crit based on Assassins touch level.
Skill: Twin fold blades - Quickly strike 3 times dealing massive damage 

At level 10, the option for this class would be to continue in the Ninja field - Become a Ninja Master (so on and so forth, learning stronger Ninja based skills.) 

So that would be a 'pure ninja' 

Now say that you do not want to focus on Ninja related skills, Keeping the ones that you have learned, you would move on to a more primary class. 

So if the ninja decided to become a master wizard, they would be Ninja > Master Wizard. You would still be able to keep all of the skills you had when you were a Ninja, as well as learn basic ninja skills in the future (say a few more passive.) But your Primary will have changed. At the next milestone (probably 30) You will be able to do the same. 

remember, if you decide to stay pure to whatever class you start as - the skills will be powerful. Although combinations of classes, Say if somebody were to start out as a Cleric (healing/buffing class) and decided to change primary jobs to a warrior or trojan(dual wielding weapon class) In a battle they would have an upper hand at healing themselves and giving themselves buffs. The promotion at 10, and 30 are intended so that people can go out on their own and solo dungeons and quests if they chose to. Bring more to the team if they are on a team. 

I hope this cleared things up.