You would want it for the same reason you would want it for any other language.
"upload and go" is a convenience feature of Heroku, not it's reason for existence. It does make it easier to deploy Ruby apps but, again, it's not the reason it exists.
Something that scales, inexpensive, and with out the hassles of system admin that is required with ec2, slicehost, rackspace cloud, and other cloud sites.