I’ve added it to this website as well and it’s now managing the placement of Google Analytics for me.
It can of course be used for more than only Google Analytics. You could place tags to measure how many people have reached your sales page and have actually made a purchase.
You can always add these codes yourself but the added benefit from using Google Tag Manager is that it’s dynamic. No need to edit your website’s templates. Just set up a tag, define on which page(s) it must be shown and you’re good to go. It’s a nice and simple concept. Free as well.