A good handful of the Simpsons ones were actually fun, or at least had funny enough linking material to be entertaining. All Singing All Dancing was actually fun, as was the Homer Simpson roast. And they made sure to lampshade the heck out of it every time. I especially liked the huge Take That at Ren and Stimpy in that one episode. The only way to make a clip show good is to give perspective on the clips. The one where they were talking about romantic backstories had a little depth to it.The writers for the Simpsons hated doing clip shows, but were forced to. They tried to make them as entertaining as possible. Yes, the second one did have new footage (though some was repurposed from older episodes, and from earlier in the same episode).
Japan does this for three reasons...Some anime do this too, like Sailor Moon. Many people thoght DiC made that episode (most countries skipped it), but surprise, surprise-it was part of the original Japanese run! Pokémon had a few that were Japan-exclusive, unlike Sailor Moon.
1) budget, obviously. No need to go into anymore.
2) to recap major storylines before a huge new arc
3) same reason they have filler episodes. And sometimes to lengthen the series by adding them at the end.
But mostly reason 1.