Doing this allows them to effectively engage in the same bad behavior an individual can, but without the risk associated with a single individual doing all of those things themselves. It also may allow them to scale better too. :/
But I think the Feds are pretty good at building cases. Since it's a crime to lie to a federal investigator, all they have to do is find some fact they can prove about you that you'd be likely to lie about, and get you to lie. (Technically the lie has to be material to the investigation, but in practice any vaguely related lie seems to do.) Apparently it's often not that hard to do; they're trained to exploit the psychology of the situation to their advantage.
And once you've lied, they have something on you, and they can use that as leverage to get more.