Chatted with Matt about this, and basically they do what they are titled - they allow you to create blocks of custom content. i.e. you could create a block that was like
CODE
<div>Special Offer!<br />
Sign up now and receive 10% off using discount code "12345"</div>
Then anywhere you want that block to show up you just add the tag in from the Insert special menu, or by using {block.block_title}
It's a basic tool to make it easier for you to add a block of content that will be reused repeatedly throughout your site so that
1) You don't have to manually re-enter or copy/paste it each time
2) You can update one "content block" which will update that all over your site.
I started playing with this feature the other day. It is REALLY useful and I am loving it to death. It is sad how so simple things have to be to amuse me.