Rudde, I think you need to be a little more specific in the features you are looking for (more than just a definition of what VPN is / can do).
For ease of use it could be an interesting feature, where DA clients have the option (if enabled by the admin) to connect to the server via VPN and either access their email (more) securely, or access the internet through one of the server IP's. If monitoring could be linked to DA (bandwidth usage, internet activity / websites visited), that would be highly convenient.
The motivation would be to provide more secure email collection and optionally (anonymous) browsing and circumvention of local (and/or country-wide) firewall limitations.
Having said that though, since allowing clients to surf through a server IP introduces risks in itself (reduced accountability for online activity/website visits/abuse, extra bandwidth use), there is also much to be said not to make this too easily available. Not sure how 'proper' VPN-providers deal with this, whether there are sturdy logging / monitoring options that give the sysadmin control and insight in what is going on.
Look forward to more discussion on this topic!