They could be a gift from an unknown shinobi who recognized Naruto's potential. This shinobi might have a special connection to a blacksmith who crafted these unique claws.
In the fanfiction, the metal claws could be the result of an experiment. A scientist or a rogue ninja might have been working on creating a new type of weapon. Naruto somehow ends up with these claws. Another possibility is that they are a cursed object that Naruto accidentally stumbles upon. But instead of being cursed in a negative way, they give him new powers. Or perhaps they are a creation of a friendly tailed beast, using its own chakra to form these metal claws for Naruto.
One origin could be a genetic mutation within the Uzumaki clan that has been dormant for generations. Naruto, being an Uzumaki, could be the first in a long time to have this mutation manifest as claws. Or it could be due to a curse placed on him by an enemy. However, instead of weakening him, the curse gives him the claws which he then learns to use to his advantage. It could also be a gift from a powerful sage or spirit that he encounters during one of his adventures, seeing the potential in Naruto and bestowing him with the ability to have claws for a specific purpose like protecting the village or fighting a greater evil.
It could be an old enemy of the Hidden Leaf Village who has a vendetta against the village for some past wrong. This antagonist could be a powerful ninja who also has his own set of metal claws, but his are corrupted with dark chakra. He wants to steal Naruto's claws, believing that they hold the key to ultimate power, and will stop at nothing to get them.
One possibility is that they could be the result of some advanced medical ninja technology. Maybe after a severe injury, the medical ninjas in the village developed these metal limbs as a replacement for his damaged ones. Another idea is that it could be a gift or curse from some powerful entity. For example, a powerful sage or a dark being might have bestowed these limbs on him for their own purposes.
Maybe it was the result of some experimental jutsu gone wrong. Or perhaps he was exposed to a special kind of metal - based chakra.
The metal claws could enhance Naruto's close - combat capabilities. They would give him an edge in slashing and grappling. For example, he could use them to quickly deflect incoming kunai or shuriken. Also, they could be used in combination with his shadow clone jutsu. One clone could distract an opponent while Naruto himself uses the claws to strike from an unexpected angle.
One ability could be the power to absorb or deflect spells. The metal of the claws might have some sort of natural resistance to magic. So when an enemy casts a spell at Harry, he could use his claws to either absorb the spell's energy, making it ineffective, or deflect it back at the caster. This would give him an edge in battles against dark wizards.
One idea could be that Naruto discovers a pair of ancient metal claws with special powers. For example, they might enhance his physical attacks and also allow him to climb walls like a ninja from the Hidden Stone Village. He could then use these claws in battles against new and powerful enemies, such as a group of rogue ninjas who are after the secret of the claws.
Well, it could be really cool. Naruto with adamantium claws would add a whole new dimension to his fighting style. He could use the claws in close - range combat, maybe even combine them with his Rasengan. Picture him slashing through enemies and then following up with a powerful jutsu. It would also change his appearance, making him look more menacing.
One possible plot could be that Naruto is somehow transported to the Sonic universe. He encounters Metal Sonic, who mistakes Naruto for a threat. Naruto, using his ninja skills, tries to communicate with Metal Sonic but fails. So, he has to use his combat abilities to defend himself against Metal Sonic's attacks while also trying to find a way back to his own world.
A metal vessel in fanfiction could be a source of a new type of jutsu for Naruto. For instance, it could release a special energy that can be used for both offensive and defensive purposes. It might also have some sort of sealing ability, which could be useful in battles against powerful enemies who use dark magic or powerful jutsu.