He could use it to unlock metal locks without a key. Just by using his magnetism control, he can manipulate the tumblers inside the lock.
He might use it for finding hidden objects. If there are secret chambers or compartments hidden behind metal walls or doors, he can sense them using his magnetism - controlling ability. He can also use it to retrieve small metal objects that are lost in a large area, like a metal - plated Horcrux hidden in a forest.
Maybe he accidentally finds out when he is near some magnetic objects in Hogwarts. For example, he might notice that he can make the metal keys move towards him without using magic spells he already knew. It could be during a time when he is exploring an old, forgotten part of the castle where there are a lot of metal objects and strange magnetic fields.
He could use it for defense. For instance, creating a huge storm to block enemy attacks or a thick fog to hide his village from invaders.
One possible application is for stealth and deception. Naruto could use the ink to create illusions of himself or other objects, confusing his enemies. For example, he could make it seem like there are multiple Narutos on the battlefield.
The ability to control sand could be used for rescue operations. If someone is trapped under rubble or in a collapsed building, Harry could use the sand to carefully lift the debris without causing further harm. He could also use it for environmental control. For instance, if there's a drought - like situation, he could use the sand to create artificial rain clouds by using his magic on the sand's moisture content. Additionally, in battles, he could use the sand to blind his opponents by creating a sandstorm right in their faces.
Well, it might develop as a result of some new magic he encounters. There could be an ancient spell or a magical artifact that gives him the power to control sand. He then has to figure out how to use it. He may start with simple things like making sand swirl around his hand, and as he gets more used to it, he can do more complex things like creating sand barriers or even using the sand to communicate in some way.
One unique application could be that he uses it to relive his training days and improve his skills even more. He can go back to his time with Jiraiya and learn from his mistakes during training. Another is that he can slow down time during a battle to dodge attacks that are otherwise too fast for him.
Well, it could change the whole dynamic. He might use it to manipulate enemies, like making Voldemort's followers turn on him. This would speed up the end of the war perhaps.
In some fanfictions, it could be through an accidental encounter. Maybe he was in a situation where metal objects started to react to him unexpectedly, like when he was near a bunch of keys and they suddenly started to move towards him. This made him realize that he had this strange new power.
Perhaps he has a strange dream where he's in a world made entirely of shadows and he can manipulate them freely. When he wakes up, he starts to experiment and finds out that he can actually do the same in the real world. He might start with small things like making the shadow of a quill move on his desk, and gradually discovers the full extent of his power.
Perhaps it's through an accidental encounter with a powerful magical plant. He tries to interact with it in a normal way but ends up influencing its growth and behavior. This unexpected event makes him aware of his unique power to control nature. For example, he might touch a withering plant and suddenly it springs back to life and starts to grow rapidly under his touch.