The Most Useful Part Of Naruto's Shadow Clones Isn't What You Think
The Shadow Clone Jutsu is the principle technique of Naruto's eponymous protagonist and has been included in both the manga and anime since its earliest chapters and episodes.
The name is both a literal translation of the technique's Japanese title, "Kage Bunshin no Jutsu," and serves to describe the technique itself. Using their chakra, a shinobi can summon one or more copies of themselves made out of solid shadow. Far from mere illusions, like low-level clone jutsus, Shadow Clones are fully corporeal. They can engage in hand-to-hand combat, use any number of jutsu, and even have a degree of intelligence similar to their summoner. However, they are also very fragile, as a single strong blow from most opponents can disperse the clone instantly.
The technique, though many are capable of using it, is actually rather advanced. In Naruto's home village of the Hidden Leaf, it's even considered forbidden. Yet in the series' first episode, Naruto steals the scroll and teaches himself the jutsu at only 12 years old, even managing to summon dozens of clones at once after just one night of practice. Naruto's ability to summon and control large numbers of clones at once not only serves as a testament to his skill and stamina but also makes him incredibly versatile on the field. However, the most useful part of this technique isn't just sheer numbers and strategy. They're a major key in allowing Naruto to progress so quickly as the series develops — for one simple reason.
Naruto's Shadow Clone let him train faster and harder
Naruto isn't the brightest bulb in Konoha's hardware store. But what he lacks in intelligence he makes up for in determination and a seemingly boundless reserve of chakra. This allows him to take advantage of the Shadow Clone Jutsu's most useful property: Any information, skills, or chakra that the clone accumulates return to the summoner upon dispersal. Using this method, Naruto managed to learn Wind Release techniques in a matter of weeks, instead of the usual months or years (via Narutopedia).
Later on, when Naruto is learning to absorb and use sage chakra from the environment in Mount Myoboku, he uses his clones to attain a perfect Sage Mode transformation (something his master, one of the three legendary Sannin, could never do) in an incredibly brief period of time. He also learns that he can keep a small number of clones on the mountain to serve as constant chakra batteries. They can focus on absorbing sage chakra while he's off somewhere else, and he can gain that sage chakra by dispersing his clones.
Using this technique, the otherwise dumb and unskilled (yet incredibly powerful) Naruto managed to surpass most of his contemporaries in just a few short years. By the end of the series, Naruto has become one of the world's most powerful ninja, and it's due in no small part to his utilization of Shadow Clones as both a strategic and educational asset.