For every two Host Collectibles you will unlock an extra feature for the multiplayer mode in this order:
Office Multiplayer Level
Storage Multiplayer Level
Raimi and Rabbit Characters
Medical Multiplayer Level
Towel Girl and Bat Characters
Helipad Multiplayer Level
Chef and Rat Characters
Raid Multiplayer Level
Dead Garden Multiplayer Level
Summit Multiplayer Level
Volks and Roach Characters
Complex Multiplayer Level
Heaven Multiplayer Level
Boss Multiplayer Level
Anna and Imp Characters
Catacombs Level