This is the GL account that is used to book the cost price of the products that are purchased or came back into the company another way (eg. returned deliveries). This is only valid if you use 'real_time' stock valuation for the concerning product. This is a property field either for product or for product category.

When does the input account ever get debited? or do I have to do that manually?

This account is credited when you receive goods - and this amount is moved to AP (via debiting for the same amount that was credited) when you enter the corresponding Vendor Bill. The amount is based on the PO cost price.