Premium only servers require a legitimate Minecraft Java account and use online-mode authentication. When you connect, the server verifies you through Mojang/Microsoft, so you cannot join with a made-up username. Your UUID is consistent, and staff can treat your account as a real, authenticated identity.
That single choice changes day-to-day multiplayer. Bans and mutes hold, name spoofing is off the table, and cycling throwaway alts takes more effort. You still get cheaters and drama, but the baseline is usually calmer: fewer drive-by griefers, more long-term builds, and a community where reputation and history matter.
The cost is access. If someone does not own Java or relies on offline-mode clients, they are simply blocked. Premium only also enables servers to lean harder on account-tied systems like meaningful whitelists, Discord role links to UUIDs, and economies or claims built around stable identity over weeks and months.
