Sometimes it happens when customer makes an order and have some important question or note about his order. It can be shipping or payment requests, problems, product change, order cancel, delivery questions. specific product questions etc.
With Order Comments plugin your customers will be able to ask any question on order details page and get a quick response from a support team.
There is a free demo version available with 1 restriction - customer can add less than 5 comments only.
For documentation, details and examples please refer to this link. You may also find a plugin for your older nopCommerce version there!