There are several reasons to select Earth Magic, but one of the strongest motive is Town Portal spell, which is among the most powerful adventure map spells along with Fly and Dimension Door. Another excellent reason is the Resurrection spell (and Animate Dead for Undead), which permanently resurrects creatures at Advanced and Expert level (while on Basic level only for the duration of the battle). A hero with a lot of spell points, high power skill and Expert or Advanced Resurrection can win almost any battle without suffering losses (for example, by Blinding the last enemy stack and then resurrecting all of their own troops).
Furthermore, with Expert Earth Magic, Shield, Stone Skin and Slow become powerful mass spells at Expert level. Slow as a mass hindering spell has especially devastating effects among enemy troops, as it reduces all enemy creatures' speed by 50%, which may change the outcome of battles against overpowering opponents. Earth magic also contains Meteor Shower which is a great multi hex damaging spell and Implosion which is the strongest single target spell in the game, capable of destroying or crippling a power stack at the beginning of the fight.