Magnesium oxide acts as a long-term source of magnesium within cells 

In nature, magnesium primarily exists most commonly as magnesium oxide
Slowly absorbed, it optimizes replenishment of storage depots.
Well-tolerated by the GI tract.
Fast absorption organic forms (e.g. citrate) don't build long-term storage levels, are quickly absorbed, and can be excreted quickly.
 
Role
Magnesium is essential for maintaining proper calcium balance in the body.
Plays a role in over 600 different reactions in the body, including energy production, muscle function, and nerve signaling.
 
Increased requirements
Stress, competitive sports, high sugar diets, diuretics, alcohol abuse, aluminum exposure, taking certain medications or during pregnancy and breastfeeding
Average need for an adult: 300-400 mg per day to reach 60-80 ng/ml D3