Job Statuses are a great way to control who can collect and/or manage job data. For a handy visual representation of what modules are visibility based on job status see: Job Status Visibilty.
Jobs should be converted to completed once works have been physically completed. This then 'hides' the job from mobile devices, stopping Users with non-financial based roles from adding job costs and times. Once a Job has been marked as Completed, only those with financial or payroll permissions will have access to the job to allocate costs and invoice.