For context, Dean dies repeatedly in that episode while the day repeats with only Sam knowing.  There is an actual reason for that to happen, which is revealed at the end and I won’t spoil the fun.  Seriously, it got to Looney Tunes level events in that one and I do not feel bad for laughing.  (I’ll make the Dean fans happy at the end of this.)

Anyway, I’m going to dive into the benefits of putting a lighthearted book/story/subplot into the middle of a lengthy series.  I call it a ‘Detour Tale’ since it takes the characters away from the main story while still making progress and eventually getting back on track.  After all, not every detour will get you lost and you’re still going to get to where you were going.  Just something came up and a new adventure was born.  That happens in real life and can happen in fiction as well.  An author simply needs to make such a thing worthwhile for everyone involved.

1. The longer you go with darkness and despair, the harder it is to have anything positive happen.  It sounds strange, but you can write yourself into the cellar by always having bad stuff occur.  Not to mention it can wear on everyone involved.  A lighthearted story can help bolster the mentalities of the author, characters, and readers.  The fact that you’ll knock everyone down again is besides the point and such a move may have a stronger impact.  At least now the readers know you’re willing to have good and bad events happen.
2. World exploration and building can be done if the event has a physical detour.  This means the readers get to see a new region with new cultures and feel like the world is an even bigger place.  This can increase the sense of scale when it comes to the main storyline.  Now it’s no longer the original locations, but this new one and possibly regions that haven’t been explored yet.
3. You can have character development up the wazoo.  This downtime gives the heroes and villains time to reflect and recover from past events.  Self-reflection can lead to a change in world view or even an alteration of plans.  Maybe a hero loses their courage or a villain realizes they’re on the wrong side.  It’s actually a little easier to have these things happen in a quieter, less epic, slower paced tale.  Especially for someone like me that does a lot of action.  Honestly, not being on the main story doesn’t mean a character’s evolution pauses.
4. Less used characters can take the spotlight for a bit.  Those that are always in the forefront can be moved to the back a bit while the heroes and villains that have yet to shine brightly can get some attention.  This is probably more for ensemble casts than a story with a solitary main hero, but you can use it to bolster supporting cast.  A bland secondary doesn’t really help a story, so focusing on them for a bit doesn’t hurt.  You might even find yourself with a new book and hero on your hands.  Another thing here is that you might have some information about the less used characters that such a story will help you reveal.
5. A final note is that you can set up events for the future books.  It’s possible that something has gone a little off-kilter with the main plot or one of the characters.  So you can use this story to shake things up in a way that doesn’t knock over the entire chess game.  For example, you need to set up a certain event and it isn’t coming out right as long as you follow the main plot alone.  This event clears up a hole or adds a piece that will be important near the end.  A ‘detour story’ can take the characters away from all of the blockage and clear the stage for a set up.

Those are the big ones I can think of and I’m sure there’s more.  A major point here is to not overlook this tool if you have a lengthy series.  Might even work on a smaller scale with a ‘detour chapter’ when you have a long, heavy book or trilogy.  Now for the other thing that I promised: