It's a solution to situations like https://en.wikipedia.org/wiki/Ross_Ulbricht#Silk_Road,_arres... , where the laptop is taken by people who (a) can legally seize it, (b) can legally search it, but (c) probably can't legally compel you to produce passwords.
(Not endorsing this usage!)
It's a very good solution for people who don't know you have something like this. Clearly if the FBI (or whatever) knew of the USB kill device, they'd take a slightly different snatch and grab approach. However if the adversary doesn't think or doesn't know you have something like this, then it can be used to great effect.
I've read that LEO in the USA specifically try to grab laptops / phones while they are unlocked.
This seems to be designed as a defense against that threat model.
If there is no due process, all bets are off and your best defense is to be uninteresting to authorities.
And even if: A kill switch can simply hide / erase some things and present some weak evidence..