Here, some questions and anwsers.
Why is there free game?
Some developers use this strategy to increase rapidly the number of users.
Is free foverer?
Normally, no. This strategy is for a limited time.
Why is there discount game?
For the same reason as free game: to increase the rapidly the number of users.
Why some games are free and others offers only discounts?
The owner of the game defines the strategy. It can be offer as free, with a huge discount or any other way to be popular.
How this service works?
The image bellow shows the information flow, from game sites to your device.