I see that e.g. in OpenGL it is possible to create a back buffer with an alpha channel (RGBA8888). What's the effect/advantage over having an RGB888 back buffer?
You can use the destination's alpha (backbuffer) as a factor in blending functions such as
This, among other things, lets you do front-to-back transparency/blending.
On some platform this also lets you have transparent windows and overlays that use the front buffer's alpha value.