I've believed for a while that Hilliary's unsecured private server was for more than just FOIA avoidance. It also served as a "dropbox" to facilitate the sale of state secrets with some degree of deniability.
I believe the Cohen raid was a similar play. Q team et al could not just hand over info to Mueller, but conveyed "if you look in the yellow folder in the third drawer, you may find something interesting."
"How do you introduce evidence into an investigation (legally)?"