A (alpha) in Cycles color picker?

What means this A (alpha)? The purpose for which it is? Can´t find any explanation on this.

Alpha is the transparency of the color or how much it allows other colors/light to pass through it. https://en.wikipedia.org/wiki/Alpha_compositing

at 1 the material will be fully opaque, while at 0 the material will be fully transparent

Yes, but how to use it in cycles. No effect on the texture-, material- nor render display in viewport. Confusing???

that’s just because the ‘color picker’ is shared all over blender, and in some parts the alpha is used. Cycles on the other way uses only the color information, as ir makes no sense for a shader to have a transparent color.

Oh, I thought that it is something similar to Blender Internal, but it has any use?