Simulation-distance is extremely important for performance, so you might want to keep it low in your server. This creates a problem: mobs by default hard despawn only if >=128 blocks away from players, and only if that's inside your simulation distance. So, mobs sometimes won't despawn if your simulation distance is too small, causing mob spawning issues.
A possible fix for that is to reduce
hard-despawn-range to something lower that fits your simulation distance. But then there's another problem: AFK spots in farms will have to be closer to the player because mobs now hard despawn at shorter distances.
This plugin fixes that by despawning mobs as soon as the chunk unloads, so that it doesn't depend on distance. So, you can keep your
128, whatever your simulation distance is set to.
Note: this plugin won't do anything if your
hard-despawn-range divided by 16 is smaller than your simulation-distance. This plugin is for server owners who want to keep the range high!
Paper has a
delay-chunk-unloads-by option that helps with the issue of mobs not despawning, but it's not exactly made for just that so it won't fix all instances of this. I recommend leaving it at its default value.