Nobody likes filler episodes, especially when they're made up of old clips from previous seasons, but when you hear the explanation from co-creator Bryan Konietzko, you might feel less rage:
We got the news from the higher-ups that our Book 4 budget was getting slashed, almost to the tune of an entire episode's budget. We had two options: 1) let go a significant number of crew members several weeks early, or 2) make a clips episode. We never considered the first option. We weren't going to do that to our crew, and even if we were callous enough to do so, we never would have been able to finish the season without them.
Back on Avatar, we had something that functioned as a sort of clips episode, though it was all new animation and really hard to make, Episode 317, "The Ember Island Players." Our heroes went to a play where they saw themselves and their tales performed by actors on stage. It was simultaneously a reckoning for the characters before they headed into the denouement, and a lighthearted romp where we got to poke fun at our own show before things got really serious in the remaining episodes. Korra's "Remembrances" ended up serving the same function, albeit with old footage instead of a newly animated play. There is about 5 minutes of new footage, wonderfully animated by Studio Mir, and a bunch of funny and touching narration from the characters. There are also some fun chibi heads and other treats in there to spice up the old footage.
If you want to read his explanation in full, you can find it on his tumblr.