I play many games. These games are made by companies. These companies have business models that want you to buy their miniatures and rulebooks. These business models can directly effect the rules of the game they are selling. Technically, two of the business models I'm going to describe are for one company. I will briefly describe the business models behind Games Workshop's main line games, 40K and Fantasy. I'd love to do multiple games in one post, but I don't want my post to be that long.
Games Workshop: 40K and Fantasy - the Codex/Army Book model.
For both of their main line games, Games Workshop follows the following model. For an edition, they release a main rulebook with all of the rules to play a game, but not the rules to allow each army to make a playable list. They then release army books that work with the main rules to make army lists. They build hype for each new book, and release an initial wave of minis with the book, followed by diminished waves later.
Pros: This method makes money. Each newly released (or updated) army sells a bunch of minis for people who hear about the new book and want to play the new army. This also keeps the game updating at a brisk schedule. GW tends to alternate books for 40K and Fantasy, so every two months (on average) a new book comes out. The game rules also don't stagnate, as players must adapt to the bells and whistles of the newest kid on the block.
Cons: One of the reasons armies sell so well after a new book is released is because of the phenomenon known as "Codex Creep." This basically says that the newest army tends to be the most powerful. While not entirely true, (few recent 40K codexes come close to the ball-breaking power of the Imperial Guard codex) each new book does tend to have some incredibly powerful abilities. This means that there is a noticeable trend towards more and more power, until the edition becomes broken.
A good example is the new Space Wolves codex. Compare it to the slightly earlier Space Marine codex. Tactical marines and Grey Hunters are comparable in price. Tactical marines must take a Sergeant for 10 more points per squad, and this Sergeant has options for various weapons. The Tactical marines can also take one special weapon and one heavy. They are limited to a squad size of 10, just enough to fit in a Rhino. Tactical Marines also have the Combat Tactics special rule, which is very handy when the squad is bring shot at and about to be assaulted. On the other hand, the Grey Hunters do not have the option of a sergeant. They can, however, take some weapons on regular Grey Hunters. They do not have an option for a heavy weapon. Three things make the Grey Hunters a better choice. First, they trade Combat Tactics for Counter Charge. Combat Tactics is only useful when a squad is in trouble, but Counter Charge is useful even when the squad is intact. Space Wolves get into close firefights, and dare the remnants of the enemy to charge them. Second, Grey Hunters can take a banner in each unit that allows them a once-per-game ability to re-roll "1"s in close combat, and it's only 10 points. Third, and most important, Grey Hunters have a Bolt Pistol, CCW, AND Bolter. This means they can fire their bolters on Rapid fire, get charged next turn, and have 2 attacks each, or 3 if their Counter Charge works. What ever happened to the good old True Grit? The end result is that balance goes right out the window in favor of selling models. However, this does keep the cycle going, so the game doesn't die.
This also means that more popular armies tend to get more attention. Take a look at the history of 40K books to see how many Space Marine variant books have been released since the advent of 4th ed. Space Marines sell, so they get more books, while less popular armies get books less often. This is actually a detriment to the game, not because Space Marines are bad, but because other armies are good. Look at the Orks. The 3rd ed Ork codex was competitive during 3rd ed, but half of the entries were utterly useless. Post a comment if you played 3rd ed Orks and unironically took Lootas with sniper rifles, or Stikkbom Boyz. Orks were relatively unpopular during late 3rd and 4th because newer books were better. Then the newest Ork codex came out late 4th ed. Very few units were useless (Big Gunz, anyone?) and the Orks not only got a well written book, but a powerful new list. Ork sales immediately increased. It's not that people didn't like Orks, it's that people didn't like the crappy 3rd ed book with its one-trick list. It took almost a decade for Orks to get one update. In that time, the Space Marines and their variants got a bunch. It's even worse if you look at the time discrepancy for the Dark Eldar.
Verdict: Yes, GW's main line business model has some serious downsides. It doesn't promote game balance, and tends to give more attention to popular armies. However, this model keeps a giant like GW going. The constant releases keep new models coming out, and most of these are pretty good. So, if you like the game you learn to deal with every Tyranid army containing the Doom of Mal'an'tai in a mycetic spore. Or Space Wolves with three Long Fangs squads with 8 missiles. Or combined Guard/Inquisitor mounted Leafblower lists. Stay out of tournaments and play locally if you don't like that.