Because it's small? Portable? Cheap? Unobtrusive? Creative? Fun?

In short, a hack?

It seemed like a somewhat pointless hack to me, but sithadmin (your sibling comment) points out some valid use-cases.

Not everything has to be productive.

Because it's the sort of thing you can tape to the underside of a desk, or slip into a ceiling plenum without it getting noticed very quickly?

Surely a normal tablet or closed laptop will look far more inconspicuous just lying around an office.

Depends on the environment. If I saw a new laptop in my office I'd notice it immediately because we've only got one laptop. A huge office with machines all over the damn place, sure that would probably work just as well.

Not in an environment with any level of decent IT management going on. Most managed IT environments look fairly homogenous in terms of gear, and anything that's not an exact or very close match will immediately draw attention.

They namedropped the Pi in Mr. Robot, which might make it interesting to some newcomers to the, erm, "penetration testing" scene.

Ditto with Kali Linux. I noticed a huge uptick in seeders/leechers on the Kali ISOs I seed, after the show came out. And if I'm not mistaken, the distro was never actually named in the show, but the background image on Elliot's computer is the Kali logo.

You can shape some case for it as an charger