Ah, I get it.

Sessions recuses himself from the investigation so he can freely investigate other things and keep his hands clean of the inevitable news that POTUS is clean and that our system is entirely corrupt.

Why is that inevitable?

Because Mueller was on POTUS's side (duh), and his real investigation concerns the flagrant disregard for law and morality in all aspects of our government that led to the investigation on POTUS in the first place. Again, duh. Fully covered in notables past.

What I think, though, is that the method that they're going to use (the "excuse"), is that in order to verify the authenticity of the investigation itself, they found errors which needed to be corrected. Errors that couldn't be corrected because they were intentionally put there in order to aid the investigation.

Basically, Mueller is going to come out with the report saying, basically, that every step of the investigation, they found crimes being committed by the deep state with the goal of removing POTUS from the presidency.

And because of that, they had to figure out why, so they had to go back and then that's when all of the rest of the shit comes out.

The public isn't only going to learn that the there was such an large operation to remove POTUS, but WHY. THE BIG WHY.