Being given immunity forces one to testify to something they have plead the 5th to. If you continue through the circle of perps you can force all the members to confess crimes on each other and they all go down, legally.
Ex. Podesta pleads 5th to something he did with Manafort. Podesta gets immunity and has to spill the beans. Now Manafort is fucked. Then Manafort pleads the 5th on something he did with Comey (ie. How he was a plant in Trump's campaign). Then Manafort gets immunity/reduced sentence to testify against Comey/etc and so forth.