Because Grieve and his rebel Tory allies believe if he can show that the prime minister’s senior adviser was plotting to suspend parliament for reasons other than those admitted in court and in the Commons by Johnson and his colleagues - namely to keep no-deal Brexit as an option rather than the more respectable motive of preparing a Queen’s Speech - then Johnson will be so embarrassed that he will sack Cummings.
This offensive against Cummings rests on three assumptions, all of them questionable.
First that the government will comply with the motion, even though sources in Number 10 say they are more than happy to defy convention and precedent by refusing to publish the messages. They would rather trigger (yet another) constitutional crisis (yawn).
Second they assume Cummings would have written down his allegedly insidious motives when plotting to suspend parliament - when it is perhaps more likely that he and his putative co-conspirators could take the “why” for granted (all the rest of us have).
And finally, it presumes that Johnson is embarrass-able when all recent precedent - MPs humiliating him in vote after vote, his senior MP colleagues saying he is offending against democracy and decency by purging 21 Tories who defied him, and so on - suggest he may be missing the embarrassment and shame gene.
Cummings may be the Deadpool of British politics - detested by some of his own side more than by his opponents, but impossible to shut down.
That said, since Grieve, Hammond, Gauke, Rudd, Letwin et al rightly see Cummings as the most important impediment to them once-and-for-all destroying the option of a no-deal Brexit, they cannot and will not cease their attempt to drive a wedge between him and his supposed boss, Johnson.
In the event Cummings were to fall, Johnson probably would too - since he would be a political blank sheet of paper absent his do-or-die commitment to take the UK out of the EU by October 31, and his sole mechanism for achieving this is called Cummings.
That won’t trouble the sleep of Grieve and co.
But it does make Johnson the human shield of Cummings, rather than Cummings playing the more conventional role of aide willing to take the bullet aimed at his boss.
These two live or die together, Brexitly and politically speaking.