I used one of the Invertek VFDs for an exhaust blower in a water plant lab. The exhaust blower was pulling too hard and noisy, The vented area has a plexiglas sliding door that would buckle inward when closed, and you had to shut down the fan just to be able to slide the door.
So the goal was to slow it down without having to figure out a mechanical means (the duct work was not conducive to modification).
In the end, the VFD was only good for a narrow range of speed control at the top end (iirc, 45 Hz and up) , the motor pulled more current than expected (according to the VFD display) but it did actually function. I was skeptical, and would avoid using them still, but there are cases where it is worth a try.