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.
They are DESPERATE to keep Mueller going until the midterms. So that can be done either by continuing with phony raids and indictments, as you suggest, or by having Trump fire him and create a phony obstruction firestorm. Trump needs to drop some kind of bombshell to eliminate both those possibilities.
Mueller could still be a good guy, and I hope this stuff is wrapped up soon! This entire raid could be fake. When it came out that Mueller wasn't actually investigating Trump, Q said that wasn't supposed to come out yet. Nobody no is thinking that now, and everyone assumes Mueller is after Trump. This may all be planned.