That is one of the most significant representations of Tolkien's own conception of magic, exemplified by the two Istari challenging each other from afar by means of spoken spells, and their very words sound like imperative orders, asking that the mountain either be calm or burst with rage. Furthermore, it might probably have to do with the archetype of spells in medieval tales (but also from older sagas), in which poems or sung incantations may cast powerful blessings or curses; and, again, the theme of songs (musicality) and rhymes play a prime role. Just consider the historical figure of minstrels in medieval courts or of ancient bards/poets, who would be even requested to memorise entire epic cycles and to recite them in form of melody.
Tolkien offers numerous examples: the tenson between Finrod and Sauron, Lúthien singing and undoing the spell keeping Sauron's fortress together, the chanting of Yavanna that gave shape and life to the Two Trees and many other cases. There is so much to explore, if one delves into the deep lore of Arda!