This is such a simple feature for YouTube to add but it is really quite powerful for channel owners! In the past someone would have to click a link or button on your site that took them to your YouTube channel and then they would have to hunt down the subscribe button and then confirm that subscription. Now someone can subscribe to your channel with just one click directly from your page!
If you have a YouTube channel you should add this button to all applicable webpages immediately! Configure your button over at the Google Developers page.
Remember it is against YouTube’s terms & conditions to offer or promote prizes or rewards of any kind in exchange for clicking on your YouTube subscribe button.