In Pokémon Go you raise your Pokémon's level and evolve them with candy. Every time you catch one you earn 3 pieces of candy for the respective Pokémon, so 3 for every Weedle for example, and 10 for every one you hatch. You can also transfer duplicates to the game's professor, basically just releasing them, to earn 1 piece of candy.

and clearly the candy is a soylent green situation of some sort