GTM is something which shouldn't exist in the first place. Adding a script to your website is something that should be going through your usual release process.

But avoiding that release process is also the reason why some deparments usually want GTM. It allows them to give GTM access to other people who can then inject stuff into your site. Once you have GTM you can let marketing / ad agencies add their own tracking points and other fun stuff to your site quickly. This unfortunately sounds like a great idea to marketing people.