McCarthy reveals why Mueller did this; he just doesn’t realize it:
“After he had already surrendered 300 pages of documents to investigative committees?”
I’d say the raid was Mueller’s way of telling Manafort to stop all this cooperating with Congressional investigators, especially giving them documents. The other reason is to get headlines that imply Manafort must be guilty of something, and that imply Mueller is doing something, with the added bonus of intimidating Trump supporters who might fear gestapo attention. My expectation is that Mueller will end up issuing a report that says Manafort and Trump are guilty, just not of any actual crimes.
Mueller is looking for a Fina Cual crime to pin on Manafort connecting a Russian business or bank however tenuously to the Trump campaign. They will be satisfied with musing Trump’s hair prior to the re-election campaign so they can peel-off white female suburban voters (that went for Trump last time).