Barring Swordfighter Form, Monster Echoes are Zelda's main method of fighting in Echoes of Wisdom. However, certain Echoes offer other abilities such as gliding. The Holmill Echo allows you to dig, which is an ability you'll need if you're planning on collecting all Might Crystals. Holmill only spawns in one region in Echoes of Wisdom, and you'll need it for the "A Rift in Gerudo Desert" main quest.