These pens date from a time when all design was done on paper, in the days before AutoCAD, when rolls of paper were painstakingly worked on for hours by skilled drafting specialists, not poured out of a plotter, powered by a PC.
Quite apart from those nasty computers attacking them on one side, their fellow pens tried to take them down too. Plastic- and fibre-tipped drawing pens and fineliners, mostly disposable, were easier to use, less messy, and cheaper. Squeezed in the middle, it looked like the steel-nibbed technical pen's days were numbered.
They never went away, though. They're still here, and are still loved and used for art and technical drawing. They use bottled drawing ink, so if the manufacturer doesn't make the colour you need, you can buy ink elsewhere. Bottled ink is cheaper too, and if you draw enough, that can be important. Most importantly for traditional technical drawing, they have a more accurate line width than anything else. If you need your 0.5mm line to actually be 0.5mm wide, a steel-tipped drawing pen is probably the choice for you.