Sometimes it's useful to have the same content block appear automatically in all pages of your website.
For example, you may want to place a registration form in the right column so people have access to it wherever they are. Or you may want to show a big button linked to your company's contact page throughout the whole website. However, it would be time consuming and inefficient to manually add this same content block to every page every time.
An example using 'Global content (after)'
Let's say I'd like to have a content block with buttons (images) linked to my Facebook and LinkedIn pages appearing on all pages of my website. And I'd like it to be placed after any other content blocks that I may add to the pages' Main column.
Here's the global content block, highlighted:
To do that, first I'll click on 'Global content (after)':
Then, I'll add my content block like usual. In this case it's a Word Processor block but it might have been a form, an image gallery, etc.
That's it. That content block will now appear automatically in all pages of my website, after any other content blocks in the same position (in the case the Main column) without me having to add it to every single page of the website. That by the way will be done for me not just for existing pages; any new pages that I add will also include that content.
Conversely, if you add a content block – or blocks, as you may add as many as you want – to 'Global content (before)' it will be placed before all the content blocks in the same position, on all the pages.