They were Very Special Issues that both Marvel and DC did in their team comics - the ones from the 80s and early 90s where the team took a break from fighting for justice and vengeance and just chilled the heck out for a while. And they were always the most charming single issues, but only ever really worked within the context of the monthly grind.
While the Avengers and the Justice League and the Defenders would sneak characterization in brief moments of quiet, the New Teen Titans really got things started with the A Day In the Life story in 1981, taking an entire issue to show how the team functioned on a day off from all the super heroics, and it became a regular feature in all the team comics. The Guardians of the Galaxy and X-Factor and Justice League all did the same thing, setting aside the one issue to really get inside the characters' heads, before they smash into Despero again.
A lot of creators were great at mixing the characterization in with the action, showing the merits of the heroes' worth by their deeds, not their downtime, but they frequently got lost in the storytelling carnage, and giving them their own issue to calm down a bit always made sense.
Unsurprisingly, Chris Claremont was an absolute master at it with the X-Men, keeping the soap operatics bubbling away with regular breaks to relax. There were extensively diminishing returns in all the bloody baseball games, but there was incredible character work in these interludes that was just as important as the big, sprawling saga.
They worked because they were a breathe between the big battles, not the norm. Plenty of superhero comics since then have tried to do whole series on the behind-the-scenes stuff, but it gets suddenly tedious when that's all there is. They had to be one-offs, or it just didn't work.
The superhero companies still do it regularly, although they get increasingly lost in the cacophony of the regular super-epics. The interlude comics used to stand out because of their relative rarity, and were still the best place to go if you really wanted to get inside Cyborg's mechanical brain.