The sage of the six paths did 2 things on his deathbed
choose a succesor
and divide the tailed beasts
he told the bijuus that they will have different names and when the time comes they will know what true power is, which is through love, the reason why he chose the younger son on his deathbed.

naruto has befriended all the bijuu and they also believe naruto was the one the sage was talking about

I'm thinking that when the juubi appears that naruto is going to call their names or something and they're going to fight in the juubi's conscious and remember naruto and naruto will lead the down the right path
and the reason they gave naruto their chakra is because it has something to do with the seal

look at the seal on the ground around the sage it has two circles and tomoes
and look at the seal where it has the circles on naruto's stomach
and now the aura around naruto looks like tomoes

the bijuus already knowing they were about to be sucked up in to the gedo mazo statue gave naruto their chakra and made sure that what the sage of the six path said about them getting led down the right path will happen.
the younger son's descendant using love to control the juubi and bijuus
while the older son's descendant controls the bijuus through power and using chakra chains and shit like that