[dmdsoftware]

What is the advantage of using service accounts instead of the "normal" oauth2 clients?

I believe he is referring to Google Apps accounts in general and not specifically Google Apps Service Accounts. Service Accounts are for an entire domain account and allows you to write batch things like deleting files for other users, etc (at least that is one purpose I've used this for in the past). It's good for those sticky situations where even if you have full permission of a file, only the owner can do some actions on it (such as delete or trash the file). Service account is a way around this.

Service accounts use OAUTH2 with the addition of providing the username in which you are accessing the file with.