Mark to Market is only used on assets who’s value goes both ways, ie assets that both appreciate and depreciate. It was used to great effect during the GFC to right down a huge unrealised fuel hedging loss. It is also used to mark up unrealised fuel hedging gains and has been used in the last couple of years this way. Accounting laws in HK and most countries (except the USA) dictate that companies must show this in their P&L. Depreciation though only goes one way and is the principal way of writing down asset values like aircraft which generally only depreciate in value.