Grease Pencil has a special set of viewport real-time visual effects that can be apply to the object.
These effects treat the object as if it was just an image, for that reason they have effect on the whole object and cannot limit their influence on certain parts like layers, materials or vertex group as with modifiers. Also unlike modifiers, they can not be applied to the Object.
Their main purpose is to have a quick way to visualize certain effects on your drawings like blurring, pixelation, wave distortion, among others.
The effects are extremely dependent on the zoom and orientation of the viewport or camera. so keep in mind that they may vary with viewport or camera changes.
Visual effects are mainly for quick viewport visualization and are not so reliable for final production renders. If you want more precision with effects it is recommended to use the compositor.