Others have defined what GTM does pretty well without a basic definition of it’s most common purpose. Google Analytics (GA4) has a number of conversions it can track automatically on your website, like if someone spends more than 5 minutes on your site after finding it in search results. GTM allows for customization of certain conversion triggers so you can track more specific actions and those that don’t have automatic tracking parameters in GA4. Using GTM just allows for more robust and customizable tracking basically, at least in its most common usage.