Scene component is declared as Actor’s root, basically any child class of Actor, its root is Scene Component.
Why it need a Root Component? Lets say the actor have some static mesh components. When you place it to the level, it need something to attach to. Without Root Component, the transform of the Static Mesh Component of the actor could be messed up, since the relative transform of the Static Mesh Component could be the world transform too, usually you will get a warning says, your Actor doesn’t have Root Component.
Why it has to be Scene Component? Because it is the most basic of Actor Component which can be rendered to the scene.
Why character use Capsule Component? Since the Skeletal Mesh is going to be animated, it need something can be the master collision of the pawn.