What's the meaning of Composer.renderToScreen

[Tao]

I’m learning post processing in threejs. But don’t understand officil’s demo from:
https://threejs.org/examples/webgl_postprocessing_unreal_bloom_selective.html

I can’t understand the render’ logic in case of ‘Scene with Glow’:

firstly, it set bloomComposer.renderToScreen = false;
then, call renderBloom, to render the selected meshs, but what the meaning of renderToScreen = false? ( I tried to switch renderToScreen=true, had differenct effect, but don’t know the reason, neither)
finally, call the finalComposer, I don’t understand this effect too. Why use shader here?

It’s something like photoshop’s making layer?

		function render() {
			switch ( params.scene ) {
				case 'Scene only':
					renderer.render( scene, camera );
					break;
				case 'Glow only':
					renderBloom( false );
					break;
				/**********************************
				***********************************
				     My question is here
				***********************************
				**********************************/
				case 'Scene with Glow': 
				default:
					// render scene with bloom
					renderBloom( true );
					// render the entire scene, then render bloom scene on top
					finalComposer.render();
					break;
			}
		}

		function renderBloom( mask ) {
			if ( mask === true ) {
				scene.traverse( darkenNonBloomed );
				bloomComposer.render();
				scene.traverse( restoreMaterial );
			} else {
				camera.layers.set( BLOOM_SCENE );
				bloomComposer.render();
				camera.layers.set( ENTIRE_SCENE );
			}
		}

		function darkenNonBloomed( obj ) {
			if ( obj.isMesh && bloomLayer.test( obj.layers ) === false ) {
				materials[ obj.uuid ] = obj.material;
				obj.material = darkMaterial;
			}
		}

		function restoreMaterial( obj ) {
			if ( materials[ obj.uuid ] ) {
				obj.material = materials[ obj.uuid ];
				delete materials[ obj.uuid ];
			}
		}

[Mugen87]

Like mentioned in the documentation, renderToScreen controls whether the final pass in the composer’s pass chain is rendered to the screen (default framebuffer) or not.

[Tao]

It’s the original effect

It’s the effect, which’s switch to ture.

I think if the only difference is use the buffer, the effect should be same, right? I don’t know the reason.

And I don’t understand what the shader’s usage here.