The only PDH's I've found are useful relate to some form of certification. In the US, I was a TCS (Traffic Control Supervisor) which required a certain amount of on the job hours plus a 2-day course. It's required if you supervise traffic control crews (especially flaggers) in Colorado. The on-the-job hours teach you how to actually do the setouts, but the in-class training teaches you the "why's".
I just completed the NZ equivalent series of courses and scored 40hrs of PDH's towards my Chartered Engineer (CPEng) application. In my case, they don't care about a specific amount of hours but rather that I demonstrate around 2 years of "progressive NZ-specific" engineering experience so I can transfer my PE here.