I read:

> How do I know River controls the reserves?

River proves wallet ownership by sending a transaction with a predetermined amount on the Bitcoin network.

The amount of the transaction is determined by taking the smallest seven digits from the largest output of the coinbase transaction (the first transaction) from the block selected for the monthly Proof of Reserves report.

Is there a specific reason why signing a message with your own key is not sufficient?

Maybe broadcasting a transaction is more immediate and the outcome is accessible without any special tool, and at the same time you are also automatically timestamping the proof? Is there anything else?