Consistency and control. Most home machines have a limited number of profiles you can roast to and you can't change them. Professional roasters are built to have infinite variability - you can roast to whatever temperature/time/etc that you want.
And consistency - a properly used and maintained professional roaster can roast the same bean exactly the same way every time you roast. Where a home roaster is going to be influenced by outside variables like ambient temperature and other things.