Customers may have a variety of reasons to show zero price lines
1. They are consuming the hours bank and need to show progress against the hour bank on an invoice without billing the customer again
2. They do pro-bono work for the client but want to display the hours that they are giving credit to the customer to prove they have provided the work
3. They have timesheet lines of non-billable resources that need to be shown on an invoice to prove the work has been done.