Before we even get going, I would like to first give say that I dont believe that we have seen even half of what Nagato is capable of.
I view "Pain" as a weapon in Nagato's arsenal.
Just because the 4th was known as "the yellow flash' doesn't mean that he was at all limited to that jutsu (obviously)
and i think nagato is no different
Now back to the main point!
Okay, so we all know that the bijuu's are massive beings made up of chakra.
And we also know that pain (or i guess, nagato) has some pretty beastly jutsus.
And all of these jutsu's take a tremendous amount of chakra.
We have even seen, in the last few chapters, that pains main weakness is just running out of chakra and not being able to use his godly haxor attacks!
Well, that is exactly where the bijuu come in!
He is harvesting them for his own person use, to make sure that he is constantly at top power!
With the use of the Bijuu and their power, Nagato would not have to devote himself so fully to attacks like shinra tensei! It wouldnt have to be a last resort, final move type of thing. He would have so much chakra, that an explosion the size of the one that destroyed konoha would be nothing. no big deal at all!
*also, we cant even begin to know how much chakra it takes to power his "pain chair thing"
the bijuu chakra would undoubtibly help here as well!