Amazon Prime Video is available worldwide, but the content you see depends entirely on your location. The US library is significantly larger than most other regions. A VPN lets you access any Prime Video library instantly. The reason for this is structural: streaming services license content country by country, and Amazon Prime Video's catalogue in each market reflects whichever distribution deals were signed there. A title that's available on Amazon Prime Video in the US might be on a competitor's platform in the UK, or vice versa.
A VPN sidesteps the geography problem entirely. Amazon Prime Video sees the IP of GhostShield's exit server, not your real one, and serves you the catalogue for that exit region. The streaming app makes no distinction between a 'real' local user and a VPN-connected one as long as the traffic looks like residential broadband — which is exactly what WireGuard with ChaCha20-Poly1305 over GhostShield's network produces.
What you can't do is bend the laws of physics: a stream from Amazon Prime Video's origin servers will always have more latency from far away. Pick the GhostShield server closest to Amazon Prime Video's data centres in the region you want, not the one closest to you.