Bevy ambient light question

bevy-ambient-light-example.rs

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .insert_resource(AmbientLight {
            color: Color::default(),
            brightness: 0.02,
        })
        .add_systems(Startup, setup)
        .run();
}

fn setup(
    mut commands: Commands,
         mut meshes: ResMut<Assets<Mesh>>,
         mut materials: ResMut<Assets<StandardMaterial>>) {
    commands.spawn(PbrBundle {
        mesh: meshes.add(Plane3d::default().mesh().size(10.0, 10.0)),
        material: materials.add(StandardMaterial {
            base_color: Color::WHITE,
            perceptual_roughness: 1.0,
            ..default()
        }),
        ..default()
    });

    commands.spawn(Camera3dBundle {
        transform: Transform::from_xyz(0.0, 20.0, 0.0).looking_at(Vec3::ZERO, Vec3::Z),
        ..default()
    });
}