Perhaps it's for exploration. Naruto is always curious about the world outside. The robots can be sent to explore dangerous or uncharted areas where it might be too risky for humans to go. They can collect information and report back to Naruto.
Maybe Naruto has some knowledge from an unexpected source. For example, he could have met a wandering inventor who taught him the basics of robotics. With his determination and quick learning ability, he then starts building the robot army. He might also use his chakra in some unique way to power or control the robots.
Perhaps in the fanfiction, the robot army is a result of some mad scientist's experiment in the Naruto universe. Naruto and his friends have to figure out the weaknesses of these robots, which could be related to chakra or some specific ninja techniques. They might also encounter some robots that can copy ninja abilities, adding more challenges to the story.
It could be Orochimaru. He's always been into strange experiments. He might create a robot army to get revenge on the village or to prove his superiority.
Maybe Naruto creates the robot army as a way to bridge the gap between the ninja villages. He uses them for peaceful purposes like construction and rescue operations at first. But then, a rogue group tries to steal the robots and use them for evil, so Naruto has to fight to protect his creation. Well, in this fanfiction, the robots could be powered by chakra, which gives them an interesting twist. They might also be able to learn ninja techniques from Naruto and the other ninjas, making them a formidable force not just in terms of brute strength but also in combat skills.
Sasuke might be initially suspicious. He would question Naruto's motives and the potential danger of such an army. But after seeing how Naruto uses it for good, he might come to accept it.
At first, Naruto might be a bit cautious. He's used to facing opponents in battle, and a strange new object like a robot could potentially be dangerous. But once he realizes it's not an immediate threat, he'd likely approach it with his usual enthusiasm. He might start asking it all sorts of questions, like 'What are you?' and 'Where are you from?' His reaction would be a mix of excitement and wariness.
Well, he might use his powerful jutsu like the Rasengan. He could also summon his toads to help him out in the battle against the army. Maybe he uses his shadow clone technique to confuse the enemy and then strike them when they're vulnerable.
Well, he might start by using his charisma. Naruto has always been good at making friends. He could reach out to old allies from his village, like Shikamaru and Choji. Also, he might convince some of the outcast ninja who have been mistreated in other villages to join him. His reputation as a hero who saved the world would surely attract a lot of people.