When using one site to create pages that are intended for embedding in iframes on other sites, you may not want the providing site's navigation or theme to appear in the iframing sites.
For example, with Themeless you can create a page with nothing on it save for an image block and a link back to another page on the providing site, then embed that in an iframe on another site.
If the iframing site is ever likely to be visited by users already logged in to the providing site, you can optionally even disable any concrete5 edit bar and panels from appearing on the Themeless page, so they don't spoil the effect on the iframing site.
When developing or testing a block, how much of its styling is dependant on the concrete5 core styles or the current site theme or theme framework?
By adding the block to a Themeless page and then logging out, you can see how a block looks with just its own CSS, with no CSS from outside the block.