The PLEK does the leveling and crowning of the frets. The guitar is mounted in the machine, it scans the fretboard and determines exactly how much material needs to be removed, then removes it. The newer machines also do other stuff like cut nut slots and nut pockets, can cut saddles, can do inlay work, etc. I'm not necessarily partial to only having a fret level done by a PLEK machine, or think it's the be-all, end-all. I had a neck reset done on an old Martin D-28 and they also refretted it and did the leveling and crowning by hand and the guitar came out amazing. Just like leveling and crowning a guitar by hand is only as good as the person doing the work, the PLEK is only as good as the person running the machine to some degree. Some just get super nutty and get all weird when people mention PLEK machines for some reason like it's some sort of unnecessary thing, but then wouldnt think twice about leveling frets by hand if the neck warrants it. Kind of like those who are anti-CNC machines and insist that everything should be done the old-fashioned way by hand, then complain about small imperfections and nuances.