At the top of Mount Lanayru there are 2 stacks of cubes and one of them is metallic and can be grabbed using the magnet rune. What is the purpose of this?
This is actually a Korok puzzle. If you are unfamiliar with this, they are the little tree people that give you Korok seeds to be traded with Hestu for inventory slots.
You will find many of these puzzles across Hyrule. To solve if you will need to move the metal cube so that the 2 cube stacks are the exact same. Once you place the metallic cube in the correct location the Korok will appear and give you a seed.