I said in this post that writing rules was hard. However, it’s also very worthwhile, and in my experience it’s best to do it earlier in the game design process rather than later. Putting the rules on paper does a great deal to move the design process along.
First, it highlights places where the game is not yet fully thought out. When I first taught Over the Next Dune, the game we’re using as our case study, it was easy to just power through parts of the game that were not yet complete. For example, the rules for how searchers interact with the edge of the playing field were literally “they bounce off like a screen saver.” Sitting down to write the rules many games later finally forced me to think about the issue in a systematic way, rather than relying on a rule of thumb and a couple of notes for dealing with unusual situations. That was the start of the current system, which is (hopefully) (maybe) both easy to understand and relatively free of special cases.
Second, it makes playtesting without the designer’s presence possible. That means more feedback, and potentially more honest feedback as well. Seeing the game through an a playtester’s entirely new lens is irreplaceable; the more playtesters one can get, and the more one clearly one picks up their vision, the better the game will be.
Third and finally, to the extent that the game is meant to be marketable having the rules is a vital step. The rulebook is one of the first ways in which new players will interact with your game. Poorly explained rules will result in people playing your game incorrectly (with concomitant frustration and bad experiences), or even deciding to put it back on the shelf without playing at all. Either way, negative reviews and off-putting discussion will result. Writing the rules early allows one to get feedback on them separate and apart from the game, which will lead to a better experience for purchasers.
Gabe of Penny Arcade passed along this excellent advice from Mike Selinker: “‘you’re not going to come in the box.'” I’ve found that there are game design problems that are difficult even detect, much less solve, until I get into that mindset and write a ruleset that others can use. Over the Next Dune’s rules are available early for exactly that reason. Writing the rules for the game helped me get it to a state suitable for more extensive playtesting, and will make that playtesting better. There’s nothing more I could ask from a single step in the process.