Adventure Together Pokémon GO with your buddy can earn you good rewards. It'll always be worth it to adventure together with a buddy Pokémon.
You'll first need to be Trainer Rank 2. Then, you'll unlock buddy mechanics for your Pokémon. With a Pokémon as your buddy, you'll be able to feed them in AR mode and walk with them as you explore in-game. Doing activities with a buddy will earn you rewards for that Pokémon.
Adventure Together Pokémon GO: How to Make a Pokémon Your Buddy
Walking with a buddy will raise your buddy level and earn you some extra candy as you walk. You can change your buddy up to 20 different Pokémon so you can get rewards for all of them.
To adventure with a buddy, you'll have to feed them berries to give them the energy to follow you. A buddy Pokémon will follow you for a few hours until they get hungry again. Switching buddies will reset the hunger meter, but you can use this to your advantage.
You'll be able to feed buddies more often this way, letting you rack up hearts to increase your level with them.
The daily maximum heart limit can be increased if your Buddy is excited. You can make a buddy excited in Adventure mode with Poffins or by completing activities. In addition to increased hearts, candy distances will be halved, making this useful for events.
Even if you just set your favorite Pokémon as your buddy, you'll notice some neat rewards just for playing.