VPN makes network traffic appear to originate from on-campus. This benefits faculty, students and staff who access resources restricted to NUNet such as site-licensed software repositories, SMTP relay servers for outgoing email, and partner servers and networks at other institutions.
NUIT recommends VPN if you use a third-party vendor for home Internet access, especially with broadband connections such as cable modems or DSL (Digital Subscriber Line), or if you travel and use a local ISP at a remote locaton.
If you access the Internet through an Ethernet connection in a dorm room, you do not need VPN.
For further information see http://www.it.northwestern.edu/oncampus/vpn/