For example, we release a new feature but it's not available for users at the same time. So we add a temp. block that says "Feature available for all users by August 30th" - and the block is hidden automatically at a set date.
Another example. We've changed a big article and want to draw attention to a new important addition to it.
For this we add a temporary block with a set ending time that announces the changes in the article and links to them and then is automatically hidden in the article in a month.