Compare and explain clear transparent vs change color depth

[linkakro]

I tried the action "clear transparent pixels" on a png with transparent background and translucid parts as well and i did not notice a change.
What does clearing transparency do exactly in relevant context, and in which context ?
How does it compare to "change color depth action" ?

I imagine changing depth would keep all rgb intact.
While the change color depth can provide a fallback color to BLEND in if i guess right.
I wondered if they can do the same but i guess not.

[xnview]

it's not the same, when alpha is 0, RGB is set to 0