1. It becomes LESS confusing as you grow in the game. Don't worry about the confusion too much right now.
There is an additional choice of job, although it's not what you think. (Or at least what I'm surmising you think.) The actual additional choice of job is Astrologian. That job, however, is locked to you until you reach Heavensward content (aka lvl 50+).
2. Should you accept a job that early? ABSOLUTELY! There are ZERO reasons NOT to accept the job. You will continue to level the pertaining class when leveling it's corresponding job. In other words, you will continue to level your Conjurer while you level your White Mage. The "job system" (lore-wise) is meant to unlock the "full potential" of a given class. As a white mage, you'll learn abilities that benefit your abilities as a conjurer (an example of such is Divine Seal, which boosts your healing power for a duration).
3. After level 50, you will not gain any new abilities for your class. They will only be learned for your job. Thus, if you continue to level ONLY your class but not your job, you will be missing out on abilities that you could have.
Is it beneficial? ABSOLUTELY! Moreover, there is an additional class that you can learn abilities from. In your case, you are required to level Arcanist to level 15 to unlock White Mage. It is beneficial to level arcanist to level 34 for another ability that you can gain. (I'm referring to the cross-class system, which allows you to use abilities from other classes.) This help you as a white mage. The additional class for white mage is Thermatauge. Although it is not explicitly required, you NEED to level that to 26. You'll gain access to abilities that are of use to you.
Let me be clear, YOU DO NOT NEED TO LEVEL ANY OTHER CLASSES AT THIS MOMENT, EXCLUDING THE PREREQUISITE ARCANIST TO 15. In fact, I generally tell new players that eventually they should (read: [I]need to/I]) level their sub-classes for additional skills, but only after they've reached max level.