A viewer on my YouTube channel asked this question
There are several reasons for this. Some of it has to do with being practical because it makes organizing the media easier. Another reason is when you need to re-edit a video.
As I show in the video below, some effects requires you to use multiple layers or multiple tracks.
Watch the video below to understand more. And check out my video courses where I teach you how to use different video editors here.
By the way, you might want to get a Shotcut Cheatsheet which contains the top most useful Effects and Keyboard Shortcuts in one convenient PDF.
Many have found it extremely useful to help them edit faster using Shotcut.