Did you ever try to put a really big IFrame on the model-driven form? If you did, and totally irrespective of why you would do such a bizarre thing, you might have seen this message:
Those 40 rows, whatever they mean in pixels, is the maximum you can get if you go with a regular web resource/iframe.
However, if you create an IFrame PCF control, you can make that IFrame as big as you want. In the example below, I can easily get to 3000px height with the PCF control:
Whereas for a “native iframe” control all I can get is 1960px:
So, why would it matter? Well, there might be a long data entry form you wanted to display there. That seems to be my scenario. But, even if that’s not a very common scenario, it’s just an interesting fact that PCF controls are not limited by those 40 rows – be it an iframe, a long text area, a huge grid control, etc.