There are several quests that ask you to gain a set amount of candy by walking your buddy. The easiest way to accomplish this is to walk a 1KM buddy Pokémon.
Which Pokémon require you to walk 1km, 3km, 5km or 20km as your Buddy to receive Candy in Pokémon Go. Why Pokémon Go's Buddy... · 1km Buddy Distance chart
My go-to is Porygon-Z. Now, it's a 3km mon and not 1km, but (at least so far) it's the only buddy I have that has the catch assist perk with a short walking ...