A VPN is useful only if you need to access resources on the campus network that are inaccessible from outside of the network, or if you have some reason to present yourself to the internet as a member of that network.
Any browsing you do on the rest of the internet is only encrypted between you and the VPN server on campus--it's still unencrypted between the campus network and the resource on the internet.
Typical uses for VPN traffic are:
- Accessing VNC servers inside the network
- Accessing shared network storage
- Managing network infrastructure (managed switches and the like) from outside the campus network.
... assuming each of those services is blocked by the network firewall, your only way to access them from outside is through the VPN. Any network that uses NAT is likely to have services inside the network that will require a VPN to access unless the NAT router is specifically set up to allow public access to them.
To more directly answer your question, you probably don't need to use a VPN if you can access all the services you need without it. The only situation where I would recommend one would be if you were accessing sensitive data that you wanted to encrypt between you and the network. However, these kinds of resources, if they are available without a VPN, are typically encrypted anyway with an SSL certificate (HTTPS).