The Purple Potion is featured in three different games.
Phantom Hourglass and Spirit Tracks
The main purpose of the Purple Potion in these games is that when Link drinks it, he will regain eight hearts. However, in these games, Purple Potion acts as the Fairy of the game. When Link dies, he automatically drinks the potion to regain eight hearts.
A Link Between Worlds
The Purple Potion can be bought at the Witch's House after obtaining a Bottle and 10 Monster Guts. The Potion costs 60 rupees and will allow Link to perform a special spin attack that damages all surrounding enemies.