Let's face it. Games are written to make money. Not, generally , to entertain the public, but to bring in dough. Fortunately, while the entire gaming public cannot be blinded by hype, you can guarantee that the hype machine can be used to coerce more than a few players into buying a duff title. In the games industry, cash is king.
Expansion packs are another case in point.
If a company has a hit, there's always the temptation to milk it a bit more, by putting out a set of new missions or levels. This isn't always a good thing. The quality of these expansion packs vary widely. Some look like they've been knocked up in five minutes, while others look like they've had some real work put into them. They also have a tendancy to be impossibly hard.