Google Tag Manager lets marketing teams manage website tracking without requiring a developer to change the site every single time another tag or event is needed.
That is extremely convenient.
You can track form submissions, purchases, button clicks, advertising conversions, and other activity with much more flexibility.
Unfortunately, it also gives people who do not entirely know what they are doing the ability to make changes to measurement.
Poor configuration can duplicate events, corrupt analytics, slow pages, or create privacy issues.
Pros
Flexible. Saves development time. Powerful measurement capabilities.
Cons
Bad configuration can ruin the data you are relying on.
My take
Very useful for serious marketing teams, but somebody competent should own it.