From what I could gather from this chapter, a job's rank doesn't seem to be related to the character's level, so It isn't really a requirement as long as you are able to fulfill the conditions to acquire the job.
It's just that It's harder to fulfill the conditions for a higher rank job than It's for low rank jobs, so you are usually required to be of a higher level in order to be strong enough and/or have enough resources to fulfill those conditions.
On the MC's case, he was only able to become a high rank job at level 0 because of his plot power "luck".. I mean, his brother just so happened to buy out all the berries from the vendors to throw a feast because the MC was starting to play, the MC just so happened to start playing on the same day as Liliane's birthday, and because was her birthday her little sister just so happened to go out to buy berries to make a birthday cake for her, but there were no berries on the vendors because the MC's brother had bought everything, all that happening on the same day as those guys from the VR chat decided to put their plan into motion to kill Liliane using a poisoned berry, but again, there were no berries for them to buy and give to the little sister because the MC's brother had bought all of them, so they improvised and decided to send the little sister to a dangerous place, so she could find the damn berries, in order to bait Liliane and kill her using the demi-dragon worms. And then all of those events lead up to the MC bumping with Liliane as she was looking for her little sister, which triggered a quest, and then the MC with his brother went to help Liliane and the little sister, and there they found a bunch of demi-dragon type monsters, then they defeated them all and saved the day, and by dumbluck the MC got a item worth 400k from the box the demi-dragon worm he defeated had dropped. Liliane ended up being a royal knight and gave him a letter of recomendation which fulfilled one of the conditions to become a paladin, before that, from the money the MC got after selling the item he got from the dropped box he was able to make the 200k donation to the church which fulfilled another condition for the paladin job, and back when they were saving the sisters, from the demi-dragon worm the MC defeated when his embryo just so decided to awaken at just the right time and place, he got the last condition for the job, that was to defeat a demi-dragon type monster having done at least 50% of the total damage.
ANYWAYS, as I said, It was all due to PLOT POWER "luck". Under normal non-MC'ish circumstances, that would've probably been impossible to become a high rank job at level 0, because a normal level 0 players wouldn't have been able to fulfill any of these conditions.
As for what jobs exactly do, from what I understood, It gives bonuses to some stats depending on the job and give skills that are specific to the job. And as he levels up using that job I believe that he will unlock new skills of that job and get stat points for him to distributed where he pleases? Not sure.