Encourages players to play as much as possible to get their money's worth.
Some games can only be played by paying a monthly or yearly fee for access to the game. These are usually massively multiplayer online (MMO) games. Other games may have an optional monthly fee that you can pay to get a daily boost in the game. Either way, having a monthly fee encourages players to play as much as possible to get their money's worth.
The game developer hopes that by playing a lot, you'll get addicted to the game and keep paying the monthly fee. They may also make it very difficult to cancel your monthly subscription by hiding the option deep in the game's menus. It's easy to purchase the subscription but very difficult to get out. For some people, the hassle of canceling the subscription sometimes makes them wait longer before cancelling, adding extra revenue for the game developers.