And then there's just the idea of being able to pull these out of the sky literally whenever you want one. If you want to try something new out real quick, it makes no sense to figure out which of your existing Sprites to use. Just make a new one. If you're a little OCD, like I am, every once in awhile you can go prune, if you really care.
I'm having trouble understanding the difference to Fly machines. If you spin up a Debian container on a machine with a persistent volume, doesn't that have everything this does? Is this about providing a layer of useful configuration/management software on top?