The Buddy System in Pokémon Go allows you to walk Pokémon in order to gain chosen Pokémon’s candy. In some case, walking a certain Pokémon a certain distance allows you to evolve your buddy! Let’s go over each purpose of using the Buddy System.
Necessary for Evolution
Evolving Feebas can be a struggle. This tiny fish from Gen. 3 is a rare spawn in the wild, and can only be found in 10k Eggs. It requires 100 candy to evolve, and you will have to walk it 5 km for each candy to gain just one candy. In order to evolve it, you must walk a Feebas 20 km. It is also the only Pokémon that makes transparent use of the Buddy System in order to evolve but we expect more to come.
(Note: If you have Feebas as your walking buddy, switch to another Pokémon, and switch back to that very same Feebas you were walking before, you will not lose your walking progress. But keep in mind that only that particular Feebas you have walked can be evolved.)
Walking Eevee is a fair bit more reasonable. Eevee are everywhere these days, especially in Partly Cloudy weather. They only require 25 candy as opposed to 100 for Feebas, and they only need 10km to evolve, half of the distance required for Feebas. And if you haven’t used the naming trick for Eevee, you won’t have to walk 10km for your first Espeon or Umbreon!
And I know what you’re thinking – Leafeon and Glaceon are Gen. 4! Yes, yes they are, but with the recent introduction of dynamic weather, it’s serious potential that beckons discussion. Back in Platinum, the two additions to the Eevee family evolved by leveling up near a moss rock and icy rock, respectively. How that can easily translate into Pokémon Go is through Sunny and Snow, respectively.
The only requirement is that the Eevee has to be level 20 in the games, but Go has seem to broke away from main game evolutionary methods (i.e. no current inclusion of Gen. 1 stones, nor trading to obtain Golem, Alakazam, Machamp, or Gengar).
Mass Candy Requirement
Ah, yes. These pokémon. The ones that require an absurd amount of candies to evolve (specifically, 400). While they can be found in the wild, their spawns are so rare, that you’ll have better luck walking them for near 400 km.
Though they are available in raids, it consumes a raid pass (which is better used for Legendaries if your concern is rewards). Using the term ‘grinding’ is very appropriate for these three Pokémon to get their evolutions.
(Hey, no one said this game is easy).
Solely Cute Factor
All of the baby Pokémon listed above can be held by your avatar in-game. However, Pikachu and Eevee are special. If you walk Pikachu 10 km, they will go from remaining on the ground to sitting on your shoulder.
Eevee will hop right onto your shoulder, no walking needed. (Note: Igglybuff floats right next to the character, but is not held. Wynaut and Azurill cannot be held.)