The free to play model usually includes daily or weekly rewards for playing the game. This reward is usually comprised of free in-game currency that substitutes paying to play. These rewards support the free player community in a game that would otherwise be overrun by pay to player gamers.
This model is obvious. Free to play games need a way to keep players who don't pay for services (skins, levels, etc.) to feel like they are still getting the whole experience. That effect of daily rewards is obvious. Without daily rewards, free to play gamers have no way of free progress (unless there is something else built into the game). There's a subtle side effect here that isn't so obvious.
Providing daily rewards keeps the game relevant. The user is rewarded for playing the game every day. Users don't want to miss out on their rewards, so at a minimum they will log in and play to get those rewards. This forces the user to play in a situation when they would not otherwise. By forcing the player to play, the game has a better chance at keeping a larger player base. This larger player base attracts users, which means more players are talking about the game. A combination of larger player base and more play time keeps the game relevant.
