There's a reason tag managers are now the de facto for tag deployment.
Before tag managers, you'd embed tags directly into your application. It could take weeks or months to deploy them inside large, monolithic apps... Meanwhile, you'd be shifting precious developer time off high-value projects. And the practice of tagging the app just added further bloat/technical-debt to your heavy codebase.
...and then tag managers became popular.
Image credit: Blastam Analytics
Now, independent of the web application code, tags could be setup, QA'd and deployed before your coffee went cold. This led to an explosion in data collection and marketing efficiency.
This efficiency is critical in the fast-paced world of experimentation...