I believe the term was born, because it just is an euphemism in the cycle of euphemisms to name naughty or adult stuff. Maybe in conjunction with the history of the internet. When it was new, people usually had no computer/internet connection at home, but might have one at work
As long as you use it as such an euphemism, meh, whatever. But if you spell it out, it is useless.
As a tag on itch, the description would read
NSFW, otherwise known as Not Safe For Work. Content that wouldn't be appropriate to be seen by acquaintances walked past your screen.
But this is the tag definition, not the marked as adult metadata requirement definition.
It is very problematic to define something as a thing that is very, very subjective. If you work at an adult toy factory or an actual adult movie studio, what is appropriate to be seen by your qcquaintances? Or what if you work at an elementary school? Could you play a 1st person shooter there?
Also, playing games at work would get you fired in most places. So any game is very unsafe for work. I stand by it. By the literal definition, nsfw is utterly useless. It is also cultural ridden. What might be "appropriate" for your aquaintances to see in the US, might get you in jail in China. What if you visit the site of your companies rival?
Effectifly, nsfw/marked as adult is the 18+ rating here. There are some horror games that have this.