It's similar to weapon mastery in the other second jobs. It increases your minimum damage and gives you a magic attack bonus. It is an extremely important skill in 2nd job. Everything should be maxed, with the exception of just 1 point in slow (by level 70).
The MP skills were removed because they were either useless (like the MP recovery) or incorporated passively (Max MP increase). Instead of Max MP increase, we now get a +20% MP skill. The massive amount of monster MP makes MP eater incredibly effective, all but removing the need for a useless MP recovery.