maybe from a writer's perspective or something, the ranking system makes it easier for readers to go "oh ok so that guy's strong" by slapping an S-rank label to them, instead of letting the reader figure it out themselves, which sounds like a cop out for the guild system imo. It also could make it easier for the reader to tell that the mc is growing in some capacity, which after reading like 50 other stories with almost identical ranking systems, it gets repetitive and i would say eliminates the uncertainty of whether the mc has actually grown or not.