A throwback is a current post of content from some time ago, typically at least days, often years.
There are two potential reasons for posting old content. One would be for the purpose of archiving older content in a newer form, the other would be to share that older content with others.
gRegor Morrill: I have manually posted some throwback photos. Typically these were photos that Facebook's on this day feature has brought up in my feed. I download the original photo from Facebook and post it on my site with a brief description, usually including the word "throwback". I link to the original photo post on Facebook and credit the person who took the photo. I've only done this with friend's photos who I know won't mind me reposting on my site.
David Shanske asked the question of whether or not a post should be created past-dated to the original content, and a separate post be created highlighting this currently. This would be consistent with content that was already on your site, that you would highlight in the present, and might be a way to encourage further create of pre-web content.
The use case for David Shanske is pre-web content. The example was 1980s era pictures.