Roll a D20

Twenty-sided-die systems like Dungeons & Dragons route nearly every meaningful action — attacking, persuading, dodging, noticing a hidden trap — through a single d20 roll compared against a target number, which means the fairness of that one die has an outsized effect on how the whole game plays out compared to a die used only occasionally. A twenty-sided die also has a wide enough range of outcomes to support fine-grained difficulty tuning that a d6 or d10 could not — the gap between a target number of 10 and 15 is a meaningfully different challenge specifically because there are twenty roughly equally likely results to spread across that range.

Modern D&D rules introduced a mechanic where certain situations call for rolling two d20s instead of one and keeping only the higher result ("advantage," representing a favorable situation) or only the lower one ("disadvantage," representing an unfavorable one), rather than adjusting the target number up or down. Rolling this tool twice and comparing the two results reproduces that mechanic directly, and it is a meaningfully different statistical adjustment than a flat bonus or penalty would be — advantage disproportionately helps avoid a low roll rather than uniformly shifting every result upward, which is exactly the effect the designers were after.