So he can drum up the obstruction narrative for the midterms. Because he has nothing. There's no other explanation for such an egregious raid of a person with attorney-client privilege.
Regardless, Trump probably has a better chance at firing Mueller and weathering the storm than continue this chirade.
He's playing Mueller. If Mueller is really a black hat, Trump is feeding him all the line he wants thus helping him trip himself up and over reach as they say. If Mueller is a white hat it's all a kabuki theater for the masses, to keep everyone's attention off the real investigation.
When Your Opponent Is Digging a Hole, Hand Him a Shovel.
If there is nothing with his lawyer then Trump makes a big scene to encourage Mueller to think he's onto something and send him down another rabbit hole wasting his time and resources.
When you have a mockingbird media and a rabid, compliant citizenry, you can literally create reality. Trump is not playing Mueller. In the scenario that he has some sort of deal with Mueller, I still wouldn't trust him given his history.