For what it's worth; just a non-technical thought from when I used to fly turbo-props:
I think of the prop pitch being like the gears on a car. To start off, or go 'uphill' you need a low gear (fine pitch). To cruise along the 'motorway' you use a high gear (coarse pitch).
On approach to land, you need fine pitch again just in case you need to go around and climb.
As others have said; a constant speed system keeps the prop RPM constant and varies the blade pitch according to how much power the engine is producing. Turbo props generally have two propeller RPM settings - fast for take-off and fine pitch, slower for the cruise.