No one likes long explanations that can be easily shortened so I'm gonna get right into it:
Uses for opacity:
-If you have a completely black sprite that covers the screen, darken it down to around 30 or 35 opacity to make it seem like the scenery is in the night or evening, or in a dark building.
-If you have the same one color sprite that covers the screen, a different color can give off a scenery of emotions( Red=Hell or anger, blue=depression, green=feeling of life)
-Make a character seem ghostly or spiritual by cutting their opacity in half maybe
-Decrease the opacity in a section to make the player realize that they need to place something in that area
-Take a white(or any color) rectangle or square and cut the opacity and place it as it was a window, so its transparent...like a window
-Water, if the player needs or wants to look into a body of water, maybe they can see some details under it so take a blue sheet and reduce the opacity and place it over the area where you want the water to be ( and add the Water effect if you want)
-If everything has lower opacity except you then it gives off the illusion like your in a spirit or dream world
I will add more if I think of any, hope this helped any new players or experienced players looking for ideas