Minions work even when you're offline as long as the server is running.
You can place as many Minions as you want on your island.
Crops and trees grow faster than in a normal world (randomTickSpeed x3).
Oak leaves have a chance to drop a Golden Apple (0.5%) or an Enchanted Golden Apple (0.25%) in addition to the usual saplings and apples.
The world remains empty outside your island, but the biome (Meadow) allows mobs to spawn normally: passive animals during the day, monsters if your area isn't lit enough at night.
Killing those mobs yourself has a small chance to drop a Biome Stone — a consumable that lets you change your island's biome chunk by chunk.