A variety of things are only available from the trader so you eventually need to find him anyway. Those include a few items and several ingredients. The quickest way to find him is to sail along the coasts of black forest biomes. He is generally near the shore in a special warded 'bubble' that protects him and you while transacting business. You will know you found him when a small 'sack' icon appears in your mini-map. He has multiple spots where he *can* appear but the first place you find him becomes his permanent unique location. Those locations are generally midway or a bit less to the world edges so it takes some searching.

You exchange the non-coin treasure for coin and then purchase things with coin. There is also a placeable item you can make using several items called a treasure chest that is decorative only and two(?) variants of coin piles to scatter around. That's the only other use of treasure AFAIK.