Forced was perhaps the wrong word...fair enough no-one is holding a gun to your head, but let me give an example.
Say a developer comes up with a game which involves online play against other owners of the same game. This dev might want to make extra money, and thus could charge extra for (example) better weapons, or tools, or whatever. As a result, to compete with people who have paid for better stuff, you will potentially have to buy things of your own... or else resign yourself to not having as enjoyable time playing a game which you have already paid for...