When the Finished Product has a component tracked by Serial Number, Odoo activates this field in order to facilitate the process of registering the consumption of the serial numbers, i.e. by adding the serial numbers in this field we are on stock.move and Odoo will automatically populate stock.move.line.

The reason this is only possible for the components tracked by serial number is because contrary to the lot numbers, to each serial number we can only link the consumption of 1 unit of a product. Whereas for lot numbers, we still need to click on the green button to manually add the lot number and the quantity consumed from each lot: