If you master all of the Matera in a single group, revisit Bugenhagen's planetarium and examine the Huge Materia of the same catagory you mastered. Doing so will get you the Master Materia for that group. Master Materia basically equips you with all - or almost all - of the spells, summons, commands or effects from that group.

For example, the Master Summon Materia equips you with every summon spell and enables you to use each spell an unlimited amont of times! Providing you have the MP to do so.