The original joke from (The Hitchhiker’s Guide To The Galaxy) was actually written in reference to early LED watches which, in order to avoid quickly burning through batteries, required the wearer to use their non-watch hand to illuminate the display…which made it less convenient than just a normal manual or self-winding mechanical watch. (They were also less accurate than a chronometer-grade mechanical, although being solid stare and encased in giant ‘cushion’ cases were substantially more resistant to shock and impact damage.)
By the time the TV series was made, the joke no longer had context as LED watches with a constantly updated display became popular, although it still probably wasn’t as baffling to American audiences as why the name “Ford Prefect” was supposed to be peculiar. I suspect if Adams were writing today he’d replace “digital watches” with smartphones and how they are enabled by effectively making the user dumber.
