Comment by JimDabell

It’s a little bit like dependency injection for websites, used by marketing teams.

The people responsible for maintaining a site don’t want to know about all the different analytics tools the marketing team wants to use, and don’t want to be involved whenever any changes need to be made. So they expose a mechanism where the marketing team can inject functionality onto the page. Then all the marketing tools tell the marketing team how to use GTM to inject their tool.