The moon effects energy as it effects the tide. While you can cast against the natural push and pull [say an emergency] most practitioners prefer to cast with the moon phase, as spell casting is easier when going with the flow.
New moon, it's seen as the peak for negative spells, or the time to begin to cast positive spells.
Waxing moon [New to full] is seen as the time to cast spells to bring things towards you. [Love, wealth, luck]
Full moon is the peak for positive spells to bring things towards you. You can also start casting negative spells or to take things away from you.
Waning moon [full to new] cast spells to take away [curses, weight loss, remove energy, loose someone]