The PS2 had pressure-sensitive face buttons, that didn't go anywhere because they were annoying and useless. I really can't see pressure sensitive keyboard buttons catching one for a similar reason. What's the use case?
Things like throttle, smooth turning in racing games, or really any vehicle movement come to mind. You could also play digital instruments potentially and actually capture dynamics potentially. There is definitely potential here, though until it becomes more popular (if it does) I don't think that potential will be fully realized!
Yep, the question is not the use-case (as there's one or two examples in most/all current games and some apps), the question is developer support. Most people don't/won't have this, so it would be more on the keyboard manufacturer to create drivers to "interpret" for games that don't have support since it isn't popular enough to merit developer support for any reasonable fraction of games. They could implement a "threshold" setting where the keypress is interpreted as "walk" with a light <33% press, "jog" for 33-66%, and "run" for >99%; so basically 3 buttons in one. Combine that with the optional actual in-game native dev support and they might be on to something.