Every post is supposed to be original- but there are a lot of people who plagiarize content out there, and they wouldn't want to use it. In addition, not everyone knows about it. There are also some posts you wouldn't want to use it on- if you're doing the seven day black and white photography challenge, for instance, most people, including me, just copy and paste the rules in below their photograph, so Original Works probably wouldn't recognize it as original.

But yeah, 99% of the time you just want to tag Original Works in the comments of every post you write.