Several years ago, for a Journalist or Human Rights Worker, it was suggested to run two exactly alike computers, I think with Whonix. One is to be Air-Gapped, and the other is online. Online one receives encrypted message, which is moved over to the Air Gapped one (USB Problems not mentioned.) Encryption and Decryption of Messages is to be handled only on the Air Gapped Computer.
There was some mention of installing OS from Scratch frequently, routinely.
I say this to partially answer the original question, Benefits of Airgapped computer (Qubes PC was the specific question.)
When US based Security Expert Bruce Schneier was reviewing the information provided by Snowden. After he did it, he wrote about how he did it. He purchased a used, refurbished laptop from a randomly chosen computer store. (They used to exist.) He removed the physical hardware that allowed it to be online. He installed and review the entirety of the Snowden Files (I am guessing including the parts that have yet to be made public). After he took his notes. He Hard Erased all the information on the Drive. Then I think removed the drive and physically destroyed it.
Only after he destroyed his copy of the Snowden Information, did he write that, - for awhile - he had a copy.