I just gave job to my level 60 conjurer, now she's happily level 60 white mage. It's just sort of like equipping (permanently) a job stone on them. (Showed in same portion if their equipment ui too)

Practical uses are, as has been mentioned, some job specific gear, and the new jobs, for people who have only leveled any of those to 60. Earlier they couldn't have 60 level battle retainer unless they leveled also some original class past 50.