The Six Signs are actually the Infinity Stones, disguised.
Iron for the birthday, like how Doctor Strange gives away the Time Stone so Tony Stark can stay alive. Bronze carried long, like how Vision carried the Mind Stone in his head all his life. Wood from the burning, like how Thanos got Loki to give up the Space Stone by burning Thor. Stone out of song, like how Thanos had to have an emotional response to someone to get the Soul Stone. Fire in the candle-ring, like how the Power Stone was kept locked away by the Nova Corps, thus it was at the heart of the good guys' base of operations. Water from the thaw, like how the Reality Stone was surrendered following the destruction of its environment.
Once Will gains all six, he gets the power to do great things, particularly in Silver on the Tree when they help cast out the Dark.