if something is a stage in a process then that something is required for that process & the formation of metamorphic rock is a sequential process, with stages of rock being compacted, heated, recrystallized ->  recrystallization is required for the process of the formation of metamorphic rock  recrystallization is a kind of event & if an event is required for a process then that event must occur before that process can occur ->  if recrystallization is required for a process then recrystalization must occur before that process can occur  recrystallization is required for the process of the formation of metamorphic rock & if recrystallization is required for a process then recrystalization must occur before that process can occur ->  recrystallization must occur for the formation of metamorphic rock to occur  if something changes to something else then that something else is formed from that something & exposure to extreme heat and pressure changes / metamorphoses sedimentary / igneous rock into metamorphic rock ->  metamorphic rock is formed from sedimentary rock  metamorphic rock is formed from sedimentary rock & sedimentary rock is made of sediment ->  metamorphic rock is formed from sediment  recrystallization must occur for the formation of metamorphic rock to occur & metamorphic rock is formed from sediment ->  recrystallization must occur for sediments to form metamorphic rock  recrystallization must occur for sediments to form metamorphic rock & formation in the rock cycle means an old class of rocks changes into a new class of rocks -> recrystallization must occur for sediments to form a new rock