Pay to Win
The issue with free apps is that we developers need to eat, just like every other human being on this planet.
Such free applications are known as "freemium." While they are indeed free, to make the business profitable, money must come from somewhere. Sometimes ads are inserted, but they can be really annoying and intrusive. At other times, players are encouraged to pay a small fee to get something in the game that might enhance their experience. This model is sometimes referred to as "pay to win," and there are three main reasons a user might pay within a game.
Firstly, for cosmetic items: clothes, poses, emblems, and other aesthetic features that don't influence gameplay but are purely decorative.
Then there are accelerators: those that allow you to speed up time in the game so you don't have to wait for a particular action to complete, like building a structure or something similar. In my view, this is the most blatant trap, as having to wait 48 hours for an in-game action to complete is nothing more than a test of the player's patience.
Lastly, there are the unlocks: paying to unlock a specific item, vehicle, weapon, or something truly necessary to progress in the game. If you pay, you gain access to the select group of affluent players who can afford to pay to keep playing (much like in arcade machines).
In the end, the mobile gaming industry is currently stuck between the free and the easy, where you can win a round of Need For Speed by purchasing nitro for your car or gain skills by buying chests in Clash Royale.
What's undeniable is that it's a very, but very profitable business model. Just SuperCell made over $5 million a day from gems and chests in their games.