If you go to an outside service like Dropbox, you’d typically set up an account with your existing email address and a password. My point is that it’s possible for people whose institutions don’t offer Google accounts to do exactly the same thing with Google accounts.
For all of those services, the service provider will typically offer a way to “roll in” any existing consumer accounts if the owner of the domain signs a deal with the company. That roll-in process at Google is exactly what causes the gtempaccount accounts to be created.
Some vendors even offer you the choice about whether you want to force roll-in all the consumer accounts or not.