I can talk about the Viscount. Ground Fine Pitch is automatically actuated on the ground. When landing this reduces the considerable residual thrust at flight idle. GFP failure requires a longer runway.
Same deal for taxi.
Newer turboprops have condition levers; I'll leave that topic to those who have flown with them.
GFP also enables the engine to run at high revs and be able to provide thrust quickly if needed for go-around after touchdown -- something the jets seem to have left behind.
As was seen with the Luxemburg accident, bad things happen if GFP happens in the air.