This helper is equalivant to a block inside the Twig templating language. It it very useful if you want to alter a template outside another template.
Example: http://jsbin.com/yepube
The first definition of a block will be the placeholder.
// application.hbs
{{#block "name"}}
<span>Default application.hbs section</span>
{{/block}}
Any subsequent definitions will be replacements for the placeholder
// users/index.hbs
{{#block "name"}}
<span>We override the "name" block inside application.hbs!</span>
{{/block}}
More info about this coming soon!