Google Tag Manager features as listed on their site:
Marketing agility – You can launch new tags with just a few clicks. This means remarketing and other data-driven programs are finally in your hands; no more waiting weeks (or months) for website code updates—and missing valuable marketing and sales opportunities in the process.
Dependable data – Google Tag Manager’s easy-to-use error checking and speedy tag loading mean you’ll always know that every tag works. Being able to collect reliable data from your entire website and all your domains means more knowledgeable decisions and better campaign execution.
Quick and easy – Google Tag Manager is quick, intuitive, and designed to let marketers add or change tags whenever they want, while also giving their IT and webmaster colleagues confidence that the site is running smoothly—and loading quickly—so that your users are never left hanging