Levels are different from Rank. I'm currently level 480ish and rank 200.
Levels are the normal level ups. Every 5 level ups you get a pack of perk cards which will allow you to upgrade existing perks, add new perks, or swap in one perk for another. I have several loadouts of perks depending if I'm using a chainsaw, bow, flame based weapons, making/repairing weapons, making/repairing armour and so on.
Rank is to do with the current 'season'. Every three months or so there's a new long event thing. You are given daily and weekly quests to get 'score' which levels you up the ranks. As you go up the ranks you get a small amount of tickets per rank, and you can spend some tickets to get items that may well only appear for that season. You get access to new items every 10 ranks or there abouts. Rank 100 is the top normal rank. If you continue to 150, you get access to a a few items that are consumables, but you can redeem as many as you have tickets for. Then you can continue ranking up for as long as you have time before the event ends. Current one will probably end mid December.
As for benefits, more levels means more perk cards to upgrade or choose from. Up to a point. Once you're past 500 you probably know and have upgraded everything you want or need to.