Just to add to this - I've just come across a very good use case for being able to select currency for draft orders.
As you might be aware, if a customer pays for a normal frontend order in a different currency to the store's default, the order cannot be edited.
My first thought was "fine. we can just cancel the order and create a new draft order for the customer to accept". Well... turns out that's also impossible. So that's the use case!
Reading through this thread it seems there are some limitations with Shopify currency converter, namely not being able to edit inside the basket?
I have been using ML Veda's auto currency converter and was considering stopping to now use Shopify's which is free. I will re-assess that now.
I shall also reach out to ML V and see if they can add some code to enable draft order currency change and report back.