The ability to play past events, allowing you to unlock limited costumes, command codes, and welfare servants (like Santa Altera or Chloe von Einzbern).
An FGO private server is a fan-made, independent modification of the game client and server architecture. Instead of connecting to Aniplex’s official servers, the modified game connects to a private, community-hosted server. Why Players Look for Private Servers
From a technical perspective, recreating FGO's server-side logic is an enormous undertaking. Unlike some games with leaked server code, FGO's backend is a closely guarded secret. Creating a functioning, up-to-date replica would require a dedicated team of reverse-engineers and programmers working for months or years.