Your familiar gains a benefit based on its creature type.
Celestial
When a creature your familiar can see within 60 feet of itself fails a D20 Test, the familiar can take a Reaction to let the creature reroll the test with Advantage. The creature must use the new roll.
Fey
Your familiar can take a Bonus Action to teleport up to 30 feet to an unoccupied space it can see. Attack rolls against the familiar have Disadvantage until the start of its next turn.
Fiend
When your familiar takes the Attack action, it can replace an attack with a 5-foot Emanation of hellfire. Each creature within the area must succeed on a Dexterity saving throw against your spell save DC, taking 4d6 Fire damage on a failed save or half as much damage on a successful one.