@@helo_tonk_here as M28 said, coin serves as a petal that is weaker than some other petals (like wing, stinger, etc) but also serves to be a petal that is stronger than some other petals (like salt, clover, etc.) so if you get a useless petal or a petal that you don't use, you can trade it so you can have some extra DPS if you don't have much.
