When you post a purchase invoice directly from a purchase order, the order no. is filled in in the posted purchase invoice header, making it easy to see what purchase order the invoice belongs to.
In practice though, this way of posting purchase invoices is rarely used:

Most companies post purchase invoices from the purchase invoice page, either by scanning or manually entering the purcjase invoice and then selecting purchase receipt or order lines to match.
In those cases, the order no. is NOT filled in in the invoice header.
This makes finding out what purchase order(s) the invoice belongs to more difficult.

It doesn't have to be though:

- The order no. is available in the purchase invoice lines, but not visible. It would be helpful to have that field available to add in the lines.

- If the invoice is not for multiple purchase orders, but just one order, it would be nice if the field in the header would still be filled in.