Scroll depth tracking in web analytics is one of those things you simply must do, especially if you have a content-heavy site.
Tracking scroll depth not only gives you an indication of how much users are digesting your content, but it also lets you turn meaningless metrics such as Bounce Rate into something far more useful.
Google Tag Manager, or any tag manager, allows non-developers or even developers to implement front-end or tracking code with more flexibility and without upsetting or possibly messing up actual code.
For example, you could easily create a Google Analytics event tag with a click or a form submission without having to program it with code. Or you could implement other pixels and tracking codes like Facebook or LinkedIn without having to deal with developers.
You can easily implement difficult tasks like tracking if a lead form was even viewed or if a person scrolled to 75% of your page. You have a lot of versatility with what you can do with a page. You can also include and exclude specific pages without hard-coding anything.