1) People reading comments where people complain about the title get an indication that it was changed, so there's less "The title looks fine to me", "oh, they changed it already" comments that do't add to the discussion in any way.

2) If only the changed portion of the title is colored, there's an indication of whether it was a subtle changing to better fit the article content (even if the title was the same as the article previously) or an entire rewrite because it wasn't descriptive enough or was just plain misleading or flame-bait.

3) It may help you recognize it from the main page if you see it later, knowing it previously had a different title.

4) This is something separate from what I asked about before, but if all comments from HN staff noting changes followed the format Changed the title from $OLD" to "$NEW" because $REASON I think that it would cut down on the fluff comments about whether it changed (as in #1) and also cut down on the comments that opine about HN's editorializing of titles which seems to occur fairly commonly when the title changes on submissions of sufficient activity.

#4 would probably help the most, but even an orange asterisk after the title in a span with a title tag with the original title would be good. I'm just against loss of information in general, and would like to know what the original submitted title was in some cases, and it's not always obvious. Maybe I'm just weird.