Why the Java version matters
Minecraft's server.jar is a Java application, and Mojang builds each release against a specific Java Development Kit. Running too old a Java version means the server won't start at all; running a wildly newer one on an old Minecraft version can cause obscure crashes that have nothing to do with your actual configuration.
Modloader support windows
Forge and Fabric both track Minecraft's snapshots closely, so support for a new version usually lands within days to a few weeks of release. Paper (and Spigot before it) historically lags slightly further behind on major version jumps while its team ports Vanilla changes into a patched codebase — if you're planning to update immediately on release day, double check your specific plugin list is ready, not just the server jar itself.