The Designer and Client Want the Game to Make a Profit
At the end of the day, game development is about money, which is the difference between a profession and a hobby.
The flow of funds for a $50 console game is that the developer gets about $8 of every $50 that is spent on his game. The retailer gets about $16, the publisher about $10, advertising about $6, the platform holder about $7, and the media manufacturer about $3
The author explains a bit about the metrics: units sold, break-even point, forecast models, etc. - basic concepts for the wholly uninitiated. Again, there are better sources.