They're often used for shed conversions. There's a small Tuff Shed in my yard that that I'm thinking of finishing out (as a minimal office) just for practice, although I'm planning on putting a larger building back there once I figure out what's permitted.Had a tough shed installed about a year ago.
Two guys came and put it up in about 1½ hours.
I could definitely see how you could finish the inside of a bigger one and live in it.
Especially a two story one.