Is there any need to keep these? I've already completed the quest where you deliver the mammoth tusk. Also, how do you turn the mammoth tusk into powder?

Not sure, can you use them in alchemy? Or maybe you can just sell them.

As far as I can tell, only giants can make the powder. Ive found no use for them aside from quests and selling them... I saved several hoping they would allow bone armor or weapons, but alas, no.

Some radiant quests may require them? Not sure..

I wish we could have bone armor that would be interesting. Its also a good idea, I thought you might be able to craft armor or have some use to them but sadly this is not the case. You could also make jewelry with them, that would be nice for a tribal or shaman like character.