Notice that closed and private jobs aren't the same. Here’s the difference:
A closed job means you’re fully finished hiring and don’t want to receive new applicants. There’s also no need to communicate with candidates anymore. Everyone is either disqualified or hired. Closed jobs don't take up a job slot. It is possible to reopen closed jobs.
A private job means the job post is still active, but not visible to the public anymore. You either don't want your job post public from the beginning, or you've received plenty of applications and don't want to receive any more. So the big difference is that this job post is not visible on your career page, but it is still accessible on your dashboard. You can share a unique private link to this job post, so people can still visit (and apply). Because it's still an active job post, a private job takes up a job slot.
Most companies publish their job posts open to the public. When they have received enough applicants, they switch the job to private and start reviewing and communicating with the candidates. When they've hired someone, they finally close the job.