General Flynn’s value to the Mueller investigations are two-fold: Collusion and Obstruction. As detailed in Vanity Fair…
Flynn knows the inner workings of the effort to obstruct justice with the Drumpf’s firing of FBI Director Comey, and he has intimate knowledge of the collusion with Russia as the Drumpf’s point man after meeting(s?) with the Russian Ambassador over issues relating to election interference. And Mueller has Flynn’s son, also indicted, to bargain with.
Daddy talks, son walks. Classic John LeCarre plot.
Flynn’s lawyer met with Mueller Monday, presumably to discuss any possibility of a plea deal. It is assumed by investigation watchers that the deal would result in Flynn implicating the Drumpf’s campaign staff in the collusion with a foreign government to influence a national election. It is also assumed he would provide first-hand testimony into the Drumpf’s discussions about firing the FBI Director in an unsuccessful attempt to kill the Russian investigation. If Flynn does all that, the thinking goes, then Mueller will absolve his son of any crimes resulting in the indictment and possible charges.
The response from the Drumpf has been a weird (so what's new?) mixture of tweets, off the subject asides and even veiled threats to Mueller not to cross certain red lines in the sand, mixed metaphors and all. From all accounts, Mueller is unfazed by those threats.
One must assume that as naive as the Drumpf is about the formal workings of government the Mueller investigation scares the bejeezus out of him. In one way or another, he mentions it nearly every day and deep into many nights, indicating it is at the forefront of his worries. And rightfully so.
The Comey firing alone was the kind of crime that got Richard “Tricky Dicky” Nixon to leave office in disgrace. With the nasty odor of a willing Russian mole in the Oval Office clinging to the Drumpf like super glue, he might even be looking at an impeachment based on treachery.