But even if you send the characters off on a quest, or other journey, an episode that just plops in the middle of the story is also an aesthetic flaw. It needs to have roots, it needs to reach backward and forward and be foreshadowed and influence what comes after. There's no substitute for complexity when trying to spin a tale out longer. Trying to keep it going while it's still the same sort of thickness is a recipe for a thin story.
Of course, some times it just needs keeping a weather eye out to see what can be hooked up. Passes through can turn up stuff that should have connected with other events because it doesn't work as local color, and the interpolated event can sprout its own shoots either way, like a new passage in a first draft.
Often needs herding into place, though.