Board Thread:Naruto Discussions/@comment-30168027-20161007222916/@comment-25643824-20161030042239

"The tailed beasts spread out through the world, free to live as they want; both Gyūki and Kurama decide to return and stay with B and Naruto. However, though they are apart, the tailed beasts' chakra fragments remain within Naruto, allowing his body to act as their meeting ground".

Taken directly from wiki, the TB vowed to help naruto whenever needed because he understood and befriended them. So naruto has the potential for a huge power boost anytime he just doesn't need it.