Ideally there should be a field for Percent Complete where you can mark an action or sub action a percent complete 1-100%. If status is marked as complete it should automatically mark percent complete as 100%. You should also be able to choose if the sub actions average should roll up to the parent action.

ex: if two sub actions are 50% complete it would mark the parent action as 50% complete.

Evaluation of action performance is a metric that is just as important as project performance.

Forecasting project completion especially if an action is a critical path in a project.

It can affect budget management.

It would improve action/task accountability, In-Progress doesn't tell a project manager if they are just starting or nearly finished but a Percent complete does.