This way, older iOS versions download the smallest icon instead of the largest one. So, instead of forcing the largest icon to all devices unless they support the sizes attribute (iOS 4.2+), we now serve the smallest icon unless @sizes is supported. In other words, we’re now using progressive enhancement instead of graceful degradation :)
When in doubt, always use this snippet. With support for nearly all iOS versions and devices, and as many different Android versions as possible, it’s the most robust way to add touch icons to your site.