Leaders are not effective if all they do is throw work over the wall expecting other people to do it. That's the quickest way to isolate yourself and is why managers often don't know what's going on in times of problems. It's also the quickest way to get employees to hate you because they view you as useless.

And this is especially true when its a task managers should be responsible for, like hiring people.

If the point is to make them have no job, then why are they even needed in the first place?

Work themselves out of a job, not necessarily delegate. The former could mean solving a problem once and for all, or putting process in place, or eliminating a task. The latter means having someone else do it.

Delegating yourself out of a job means growing your report's skills so you can do higher level things, not so you can get back to doing your old job.