Opacity is fun when you want to mess with transparency, or if you're just trying to line over a sketch. How often do you mess with transparency? I usually only lower my opacity when I'm drawing a sketch. But if I'm doing space art I like to play around with it to see all the different effects I can make!
I only use it when I am lowering the sketch layer to make the line art. Very rarely do I mess with it outside of that.
I mess a lot with the opacity when I am shading. Sometimes I shade too dark, and with the lowering of opacity, I can make the shading lighter.
I lower the opacity when shading if the colour turns out too dark. (I use a specific colour when shading, then multiply the layer). I also used to do this when creating lineless art before I discovered the watercolour brush.
the only time i regularly mess with it is lowering the opacity of my sketch layer....... but sometimes what im drawing requires special "opacity layers!" things like glass or see through material require me to make a whole new layer just for the opacity-ed thing and its hard but its so fun to see the final result!!
