what is their use??

Later you can buy rare items using that. Most of them will be used to fill your third slot, (First Slot-Main, Second Slot-Sub, Third Slot-Item).

ohh ok, i guess ima hav to get a lot then

allow me to add to jadephoenix's post; you can trade Demon Fangs as early as Agata Forest.

but the best stuff comes from waiting until later, cant remember where though.

From the Emperor.
An easy way to get Demon Fangs is to press X a ton of times during the blank loading screen. On the other loading screen you have to time your x button to make ALL of the paws big. Then instead of seeing Issun, you see a Demon Fang. Hope you die. ***

I mean, hope this helps.

Saving them up is better than spending them right away because you get a special tablet that allows you to walk on water.

i jus did the floral finishers on the enemies to get fangs, easiest i found to get em, theres only a handful of decent items you can buy with them