I think the general case for it is that VPNs are typically centralized. If the main VPN provider is compromised it stops becoming a "Private" network.

A decentralized system doesn't suffer from that problem so it enhances privacy and anonymity. Moreover decentralization should mean that it'll be more resilient to any one node going down. Finally with most VPN providers you have to trust that they aren't monitoring or selling your data. If this is done in an open source way that won't be required.

It's why I find this project exciting. VPNs are the kind of thing that can really harness the potential of a blockchain.