Is it necessary to install Graphic card for using FME Form (Desktop)? How it impacts the performance?

Very good question. I don't know the answer, but…

I can't think of any Transformers that use the graphics card to do the calculations, even though some analyses could benefit maybe.

The inspector shows the 2D/3D output. Most times it's pretty basic so it will help if there is some graphic capability, but only if you need to review the 3D results. You could also write to file directly so FME would never show any 3D data and never need a Graphics card.

My graphics-card is pretty good, but that's because viewing 3D scenes in ArcGIS Pro has much benefit.

I would generally recommend it, though I cannot say for sure how much of a benefit it would make if you are using the machine exclusively for FME. If you have a good CPU and it is a monetary decision between more ram or a graphics card, then maybe look to improving the ram first.

I know that some setups allow the graphics card to help with non graphic processing if there is nothing graphical to process. I do see use on my graphics card when running certain workspaces, but it is minimal (<20%).