My guess is that the "why" is largely just showing off, coupled with oddness.
If you have a "house power" 12V battery bank, and a regular laptop which expects 110-240VAC input, 19VDC output (example, random Dell) power brick which is being fed from a small true sine wave inverter, like 12VDC-to-120VAC, you're losing energy to heat and the DC to AC conversion process.
You're converting DC to AC and then the laptop is converting that back into DC. Ideally you do something like wire up a small inline fuse and a tiny meanwell DC-DC converter that can take 12VDC input and output 19VDC natively.
He has basically replicated on a limited budget and in a very small space on land a setup that is not that different than economically built DC power systems are done on boats.
Also, I believe he hoards the power bricks as he notes.
What he does is not ideal, and I don't share the same view with him, but I understand what he does.
Except the literal power, apparently!