WordPress makes it easy to export content from one site and then import it into another. This is done through the import/export setting, which you can find in the TOOLS menu in your WordPress admin dashboard.
But before you can import or export all that fancy data, you need to create those new posts and pages. Designers and developers know what a chore it can be to constantly create content just for the purposes of testing the site before it goes live. The developers or WordPress know this too, which is why they created the Theme Unit Test.