Well, Law and Order, Trump edition.

You don’t get a search warrant unless you have reasonable suspicion of a crime, and evidence for it may be found at the place searched.

“Mr. Manafort, we have X, Y, and Z showing (whatever crime). We can put in a good word with the prosecutor so you don’t spend thirty years in Club Fed if you give up the goods on (so-and-so).”

Very much like taking out a criminal organisation. Grab someone low on the totem pole, and flip them for someone higher, rinse and repeat.