The first advantage is popularity. Although there are infinite number of ways you can use to resolve the results of an action, none is more prototypical to D&D than rolling a d20 (and maybe to RPGs in general), and rolling high. The d20 is ingrained in pop culture as the D&D (and RPG) die - if you're familiar with d20s, you're probably familiar with RPGs, and vice-versa. And, if you aren't, “rolling 20” is in memes, t-shirts, magazines, and even the urban dictionary. D&D popularized the d20 as no other game had done before or after, and every edition has used it as the main dice, making compatibility a bit easier to work out with the d20.
|The prototypical gaming die.|
The d20 also wields the greatest number of possible results without using multiple dice. Sure, there are d30s and others, but they are a bit rarer, more expensive and harder to use for percentages (each number in the d20 has a 5% chance of coming up, against 3,333% on the d30).
Using a single dice make things a bit easier to calculate: it is easy to realize that you have a 30% chance of rolling 15 or more on d20, but most people (me included) cannot say the exact chance of rolling 15 or more in 3d6 without looking it up. This also means that you always know what difference a +2 bonus or similar will make.
This five-percent increase from one number to the other is a level of granularity I can get behind. In most of the things I buy, for example, I could notice a 5% discount, but would hardly care for anything less. A “natural 20” comes up 5% of the time, which feels quite good to me as a chance of scoring a critical hit in a fight (more than that is too often, less too little - depending on what a critical hit means to your game, of course). For the level of detail I appreciate in my games, the d20 seems like a good fit.
Another advantage of using a single die is speed. Adding dice, or even looking for 5s and 6s in a dice pool takes a few moments (for me at least), and when you are rolling dice for large numbers of characters at the same time, using a single die for each one seems like an almost unbeatable method.
It may seem like I'm trying to sound “objective”, but I am aware some of this is just rationalization of my personal preferences. Anyone could come up with lots of reasons to use a d6, 2d10, dice pools, percentile dice or whatever, and there are surely some downsides to using the d20. We will get to them in the next post.