Upon meeting certain requirements, different costumes become available in the basement of the homestead for you to wear. Pretty sure they're purely cosmetic, since there is no armor like in the previous installments.

Outfit	How to unlock
Achilles' Original Outfit	Complete all Achilles' Homestead Missions and Interactive Conversations.
Altair's Robes	100% Sync on all story based missions
Assassin's Robes	Unlocked through story progression, Session 5. Different colors can be purchased from general stores.
Captain Kidd's Outfit	Collect all 24 Peg Leg Trinkets, 4 Map Shards and solve the riddle on Oak Island.
Ezio's Robes	Purchase in U-Play store with 30 points earned from in-game events.
Native American Outfit (Connor's pre-robe outfit.)	Collect all Feathers.
Prisoners Outfit	Story progression, Session 8.