its not for the spell its so that she can concentrate her powers
Azarath Metrion Zinthos: Main incantation and mantra commonly utilized by Raven to help focus, control and enhance her tremendous yet dangerous mystical powers, usually followed by projection of visible dark energy and any of a vast number of feats; if Raven is prevented from uttering this incantation, at times she apparently is unable to activate her powers; also used by Raven and others in meditation.
from the wiki
Azarath Metrion Zinthos: Main incantation and mantra commonly utilized by Raven to help focus, control and enhance her tremendous yet dangerous mystical powers, usually followed by projection of visible dark energy and any of a vast number of feats; if Raven is prevented from uttering this incantation, at times she apparently is unable to activate her powers; also used by Raven and others in meditation.
from the wiki