The steal materia comes very handy when it comes to steal the good equipments and items. You don't want to miss out those important equipments early in the game like the Hard Edge, Striking Staff and others valuable items. Also you can steal the accessory Carbon Bangle from those robots in Shinra Building which is a plus do to its awesome stats buffs and materia slots. Keep that in mind to steal from bosses and regular monsters to improve your characters with those items you steal from them. This is my personal tip from me to you.