This series of articles is a rant about mobile game design. It may extend to console and PC gaming, but I only talk about what I experienced through most games featured on the iPad app store in the past year or so.
The upgrade mechanic is found in more and more iOS games. In Clash of Clans or Birzzle, the player spends coins to upgrade something. In Mother of Myths and Star Wars: Assault Team, the player must use a specific category of loot (e.g. spells for spells in MoM). In Plundernauts, the player must follow recipes using specific items previously looted in battle (e.g. 3 Poltergas for a Poltergas Tank). In Mutants, the player can breed two mutants of type A and B together to make a stronger mutant of type A and B. Finally, in Birzzle, owning more birds increases the play time, and the sum of the bird levels provides a score bonus. The only purpose of this mechanic is to make players buy birds with real money - not to make the game more interesting.
This kind of upgrades is not new, and not exclusive to mobile games: UO had crafting in 2000, Ragnarok Online gear could be upgraded up to ten times with materials in 2003, Diablo 3 has it nowadays with gems and enchanting, and some games in the Final Fantasy series must have had something close to it as well. It seems that iOS developers have just realized that they could use this decade-old mechanic everywhere they need players to grind - and therefore pay real money to skip the grind.