GTM is a platform to manage analytics and other tracking elements. Benefits include:
More efficient way to manage Google Analytics
Adds much more flexibility when it comes to adding custom GA tracking components (events, goals, etc.)
Manage all publisher pixels via GTM (retargeting, conversion, call tracking, etc.)
GTM can easily be installed with a small script added to the of your site's source code. All configuration is then done in the GTM dashboard.