Purpose of Collectables?

[Ruin]

Never played the previous AC games - it was the pirate element that drew me to AC4 - so I'm curious as to what purpose the Collectables serve...

I've received quite a few so far, but have yet to see anything to indicate what exactly they do... I had presumed they would decorate my hideout or some such - much like I presume the purchasable Works of Art do (but am yet to buy one to try) - but I have as yet not seen any...

Am I missing something, or are they merely an achievement thing like the Animus Fragments?

[Ashmedai]

I'm honestly not sure what the Animus fragments do, if anything. The stones you collect from the Mayan Stelae unlock a set of armor if you collect all of them, though.

[Xanthatemra]

i know fragments in the past have unlocked other story elements, they give you other unlockables. i dont know about this game yet havent gotten into to much with finals week upon me. but i like i said in past they just there to collect for 100%. i think brotherhood gave you background missions of ezio and his girlfriend so like back story and the others were just other costumes and such

[Jalamenos]

The Animus Fragments is just a collectible for an achievment.
The Manuscripts and Letters in a Bottle don't really serve a purpose unless you really want to read them for some additional history.
The Shanties (flying pieces of paper) are songs that when collected the pirates on your ship will sing while sailing (can be turned off).
The Mayan Stones and Templar Keys both unlocks armors if you collect them all.
And in the present-day Abstergo collectibles it's just snooping and won't make much sense if you haven't played the previous games.

I think (not sure) that the pieces of art you can buy shows up in your hideout once it's unlocked.