But on the other hand, every once in a while someone on screen will say something that just makes you go "huh?" Sometimes, you can rewind and listen again -- and often as not, still have no idea what they're saying. So you've got to go back again, turn on the subtitles, read the subtitles for that line, turn them back off... and now you've already ruined the flow of whatever you're watching with all the back-and-forth and re-configuring you've had to do just to hear one throw-away line.
Hence, retroactive subtitles: press a button on your remote (or keyboard or what-have-you), and it shows you the subtitles for what people just said in the last half minute or so. Then they vanish a few moments later, once you've had time to check what you missed, without ever interrupting what you were watching.