Characters level up faster.
Description
Level Up is a support ability in Final Fantasy IX that every permanent party member can learn. It has the wearer gain 150% the normal EXP from battles (rounded down) for the cost of 7 Magic Stones, allowing the character to level up faster.
Obtained[]
Level Up is learned from numerous equipment pieces for variable amounts of AP; Vivi learns it the fastest at 30 AP, while Zidane learns it the slowest at 75 AP. Equipping multiple equipment pieces that all teach Level Up, masters the ability faster.
The earliest source is from the Iron Helm for Steiner and Freya, which can be stolen from the Plant Brain in the Evil Forest, and found in Dali Underground storage area 
. An easy source for everyone is from the Fairy Earrings found in Fossil Roo. The player can also buy it from the Treno Auction House at any time and synthesize them. The party can also get a free Extension (for female characters) that teaches Level Up from Moguta in Gizamaluke's Grotto by gifting him a Kupo Nut after the party returns from the Outer Continent.
Use[]
With Level Up, the wearer gains more EXP from battle. Though it may sound good on paper, Level Up may not end up being worth prioritizing over other support abilities. Bosses give no EXP, and so there is no point equipping it for them, and for dungeon-crawling, it may end up only being couple extra battles' worth of EXP to wear Level Up all throughout.
For any dedicated level grinding sessions, Level Up is a priority. For min-maxing purposes, it is preferable to keep the party on a low level until all permanent members have been recruited, however.
Level Up is not needed to complete a casual playthrough; the player can simply go through the game at their own pace without grinding and end up strong enough at the end to beat the game. Therefore, if short on Magic Stones, Level Up can be one of the first abilities to unequip to free up stones for other abilities.