It would depend on the job.
Set it at quote if it is still in quote stage (may not be used much depending on the company)
Set it as Completed when finished
Set as Planning when it has been planned\scheduled
Set as Open When creating and waiting for all the details
You can then on the main job screen filter by these statuses to see jobs relevant to you (i.e. you only want to see completed jobs)

It is up to you which ones you use and is mainly useful if you have lots of jobs on the go