FAR Part 121.651 (c) (4)

[ftrdispatcher]

Can someone explain this part?
It pertains to when a pilot can go below the published DA/DH or MDA.
When the aircraft is on a straight-in nonprecision approach procedure which incorporates a VDP, the aircraft has reached the VDP except where the aircraft is not equipped or capable of establishing that point, or a descent to the runway cannot be made using normal procedures or rates of descent if descent is delayed until reaching that point.

If the aircraft is incapable of establishing that point, why is it flying in IMC and attempting to make an approach? If it cannot make a descent using normal procedures, shouldn't it fall in the category of an emergency?