Within the hidden cave, Yagura, the Mizukage known for his coldness, revealed his true feelings for the first time. It was clear just how deep the bond between him and the Three-Tails ran.
In addition, there was some secret knowledge about Tailed Beasts and Jinchuriki that needed to be explained here (just my own analysis, please don't flame me).
Jinchuriki were special beings who fought by drawing on the Tailed Beast chakra within their bodies. As long as they could completely control the Tailed Beast inside them, they were perfect Jinchuriki! Perfect Jinchuriki could be divided into two types. One relied on power to forcibly control the Tailed Beast, such as Madara, Obito, and Nawaki. They could use the Rinnegan to forcibly suppress the Tailed Beast's consciousness and manipulate it like a puppet. The other type came to understand the Tailed Beast and eventually formed a bond with it. Yagura, Yugito, Killer B, Naruto, and the others belonged to this type.
Once a Jinchuriki and a Tailed Beast formed a bond and completely trusted one another, the seal within the Jinchuriki's body had basically lost its restraining function. The Tailed Beast's chakra could freely enter and leave the Jinchuriki's body. At that point, the seal had become a special space that housed the Tailed Beast. That was why perfect Jinchuriki could instantly complete a full Tailed Beast Transformation, or even summon the Tailed Beast outside their bodies to fight alongside them!
In the anime, whether in the movies or the main story, Naruto had summoned the Nine-Tails outside his body to fight independently more than once. At the end, Naruto had even intended to set the Nine-Tails free, though the two ultimately reunited through their bond.
All these plot developments showed that the Tailed Beasts within perfect Jinchuriki could leave