Per-app VPNs have existed for some time, and their use case is different from system-wide VPNs. Typically, per-app VPNs are used when you want to grant specific applications access to resources on say a corporate VPN, without granting all applications access to the VPN. Per-app VPNs are also useful if you want to protect a specific app's communications through an encrypted tunnel without affecting the rest of the system.

More info on the purpose of per-app VPNs: https://support.apple.com/en-us/guide/deployment-reference-i...