I often find myself asking "who do you think you are?"

I think it's pretty presumptuous and can be borderline arrogant when someone "apologizes on behalf of all [fill in the blank]."

On the other hand apologizing FOR others can be encouraging and help foster reconciliation if not offer some sort of start for healing at the very least. (i.e. apologizing for when someone flies off the handle and wrongs another; "I'm so sorry that just happened")

I gather the former is usually a means to save face for one's own sake while the latter shows sympathy for a wrong doing.