He had the same watch, and the backlight is so horrible. I have fond memories of him holding it up at an angle to check the time.
I only knew him for a few years of my life in early childhood. We'd go on long walks together in the late afternoon. He'd check the time when it started getting dark so we'd be back home in time for dinner!
My grandfather didn't wear the watches, but gifted them - he was an engineer from slide-rule times, and I know really appreciated compact, accessible, functional electronics. He LOVED Sharp Wizards.
From him, I received a Casio Databank, and two calculator watches for which I don't remember the brand.
So - one of the reasons I wear a Casio today.
As you can see by the small text above and below the screen you pushed F1/F2 to select the scientific operation you wanted and then "exe" it. I don't recall how "inv" worked, if you selected sine and then inv to arcsin() it or if you tapped inv as a modal and then "exe" while its inverted.
It really wasn't any harder to use than any calculator watch ever shipped. I imagine no one under age 40 has ever used a calculator watch but they were pretty easy to use, just imagine a basic phone calculator app on your wrist but easier to use tactile because of the keyboard.
Its primary downfall was 1) being very expensive compared to carrying a HP calculator in your pocket 2) the buttons and general construction were extremely delicate, the body was just chrome plated plastic IIRC.
Watches in the 80s were like phones in the 2010s it was assumed you'd only get a couple years use out of them before "having to" replace them due to poor components or style, so there's no reason to ship a long term reliable one. These were not "Rolex" lifetime purchases LOL.
Watch bands were also designed to rapidly fail, kind of like fitness bands today.
Watches in the 80s were a very consumerist hobby. Consumerism changes the individual items but the pattern never changes.
Extreme sensor watches were similar to extreme calculator watches but in the 90s. I had a DPX-500, very nice. The barometer drifted too much to be usable. The compass worked very well. As an "outdoor" watch it was fairly durable, rather like desk workers buying "offroad" SUVs that are never driven offroad as the only way to buy a reliable car.
If you think about it instead of pulling out your phone to calculate something back then you could use this