Another function of project phases is the ability to use them to group related tasks and product lines together to invoice them as a phase.  From a project's Quick Invoice dashboard, click Invoice Phases from the Actions menu.

Select the phase or phases you want to invoice and click Save.

This will then display the billable task and product lines that are in that phase.  

These can then be invoiced as per the normal invoicing options.   If the phase is selected on the project details page, the profit and progress widgets will now display the individual phase's actual vs forecast.