Trumpets serve the most basic of functionality in many games.

Believe it or not, You will use a Trumpet.
The game just inherently gives you it so it enables you to talk. And it's relatively cheap too, in case you run out. I don't have to explain why they exist.

Considering that we all have infinite inventory, It's a filler item that enables us to talk with everyone a specific time. No real issue to sell your "Speech" for less than 10coin.

Now imagine if Trumpets cost gold? That'll be something.