The developer explain how buddies work in an earlier blog. "You will be able to pick your favorite Pokemon from your collection to become your buddy, opening up unique in-game rewards and experiences. Buddy Pokemon will appear alongside your Trainer avatar on your profile screen, adding helpful bonuses such as awarding Candy for walking together. You’ll also have the ability to change your Buddy Pokemon at any time."
Today's update also patched in support for Pokemon Go Plus, a wearable device scheduled for release next week. During Apple's iPhone event last week, we learned that the game will be released on Apple Watch by the end of 2016.