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Marie went into extensive detail about the benefits of being an adventurer, and there were many of them, first of all they can sell monster carcasses and more for a better price than what they're bought outside for some reason, and also, they can buy these same materials and other products such as potions, equipment, and more for a decreased price, the higher the rank the higher the difference of prices, apparently. Aside from that, there is also the ability to pick the quests dependent of your rank, and also the ability to enter dungeons that the guild or the nation owns, I don't know about dungeons away from their jurisdiction but that's just no man's land, at that point they don't care, nor they have to know.
Check "The Protagnist's Sister Is Actually the Strongest"!