One folder, three designs
Variants build the same content in other themes, each at its own address.
Choosing a design is easier when you can see your own content in it. A variant is the same site, built from the same folder at the same moment, in another theme, at its own address: foldocs.com is also journal.foldocs.com, a blog, and index.foldocs.com, the folder tree.
A banner on a variant says what it is and links back. Variants share the original's photos, and ask search engines to stay away, so they never compete with the real site. A theme gets them by having a variants folder: see Design and themes.