I can’t really follow the details of this stuff as much as I would like. The outlines are generally clear to me but the exposition in stories is often very messy and hard to follow.
This one seems pretty clear, though. I think the basic concept is that Mueller spent 3 years investigating and only partially expected to find anything. He wasn’t really investigating because he knew there wasn’t anything. But he spent 3 years basically dithering.
Why did Mueller go to all the trouble? Because at the end of it all the core of the plan — from the beginning — had been to get President Trump in a room to answer questions and then to spring a perjury trap.
The entire Mueller investigation — 3 years of it — was just a very long perjury trap.
There is more, of course. Much of the point of the Mueller investigation was to gain the House for Democrats. In that, it was successful.