Quality control - editting, proof-reading and playtesting - is an extremely important step of commercial game production.
When a player or game-master buys a commercial game, he or she expects it to be perfectly readable and playable as-is, or, in other words, worthy of the money paid to purchase it. Without adaquate quality control, errors and ommission would inevitably creep into the product, making it less comfortable to use, or, in extreme cases, render it unusable in its published form.
Masses of errata can kill games; this has happened in more than one version of Traveller, for example (MegaTraveller and Traveller 4, I'm looking at you), where otheriwse excellent games were greatly hampered by poor editing and, in the case of Traveller 4, the ommission of an important table from the ship-design sequence. A good proof-reader or enough playtesting would have avoided these problems.
So, when you set out to produce your own commercial gaming material, pay attention to quality control measures. In other words, have a playtest, listen to your play-testers, and use a human proof-reader (rather than simply the word-processror's spell-checker). These simple measures would ensure that your game will turn out as good as it could and should be.