Uh, anything?
Say you want to acquire property for cheap, like a port or a rezoned development. But laws get in the way. You call in Reliance and demand a favour, then they'll demand a favour from the government. It's happened very often in India to not notice. An affiliate of one of the funds in question actually owns port property in India. Private Equity has a lot to gain from this deal, and not just returns.
What about the rest of the dozen investor's including private equity and sovereign funds that have invested in Jio. What political favors could they possibly gain? I think these investor's are mostly looking at multiple rates of return when Jio will IPO either in India or overseas (probably US). It is just a super attractive investment opportunity that is similar to any Silicon Valley High Growth startup.