Notes:

If you want to have a global area editable on the front end you may want to check out the excellent (and free!) Global Area block.

As the Scrapbook has been removed in Concrete 5.5 this block is unlikely to be further developed.

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This block allows you to put an entire scrapbook somewhere on your site, contained within a single block.

Maybe you have two or more blocks that you want together and in a specific order throughout your site such as sidebars, footers, etc. You could put a Scrapbook Display block on multiple pages all pointing to the same scrapbook, and you can make changes (add/edit/delete/rearrange blocks) in Dashboard > Scrapbook and they'll be automatically shown throughout the site.

In other words: this block effectively gives you the power to create Global Areas anywhere you can put a block. 

Note: there appears to be a system issue so that Auto-nav blocks set to "current level" or "level below" do not work as expected when placed in a scrapbook (ie they show pages relative to the scrapbook, not relative to the page the scrapbook item is placed on).

On the surface, that's it. Easy and useful. If you want to dig deeper...

Nesting
You can create a scrapbook (eg Scrapbook1), then create a second scrapbook (eg Scrapbook2) and put a Scrapbook Display block that points to Scrapbook1 inside of Scrapbook2. Now you can use a second Scrapbook Display block (pointed to Scrapbook2) on your site to display the contents of Scrapbook 1 and 2, all in the same block. Is your head spinning yet?

Note: do NOT put a Scrapbook Display block inside of a scrapbook that it points to. If you're nesting, no two Scrapbook Display blocks in the chain should point to the same scrapbook! Doing so will result in an error message in Edit Mode or on the Dashboard > Scrapbook page.

Other Uses
Also a good building block if you want to create a custom template to wrap your scrapblock's blocks to solve a presentation issue, use a jQuery plug-in, etc.

Lots of possibilities.