Sidekicks are important for a lot of logistical reasons. You can compare and contrast them with the protag for character development, they can reveal the protag’s interior for clarity, and they are pretty handy for getting into trouble and causing escalations.
But they aren’t just plot tools. Remember this part?
Even though Finding Nemo isn’t the story of Marlin and Dory’s friendship, Dory’s contribution is enormous. She doesn’t only function to keep Marlin from talking to himself the whole time. She delivers the emotional goods.
Her personality and voice are very different from Marlin’s. The conflict between them is built in and constantly works to build and reveal his character and create possibilities and complications.
Imagine Finding Nemo without Dory. And she doesn’t even make the logline.
Not to say that every story motor runs on a sidekick relationship, obviously, but a good sidekick works so hard for you that they are always worth considering when you find yourself stuck for story with a solitary protag.