Advance the plot and increase the intrigue.
Best Example: LOST
Three seasons in, it was painfully clear that the flashback technique that LOST mastered and popularized in its early days was beginning to suffocate the series. Creators Damon Lindelof and Carlton Cuse were straining to deliver character-centric flashbacks that resonated and enriched the castaways' backstories. Flat tales like the story nobody really wanted to know about Jack's (Matthew Fox) tattoos, Locke (Terry O'Quinn) being a gullible fool for the 47th time, and Sayid (Naveen Andrews) facing a personification of his shady past yet again left the once rapturous audience bored, while others were saddled with increasingly ridiculous additions to their bios (Kate was once married? Desmond was a monk?).
So how did Lindelof and Cuse restore the hype? By abandoning their bread-and-butter format for a bolder one, and setting an end date for the series that would effectively kill all wheel-spinning in favor of renewed momentum and the promise that all of the island shenanigans would build to an actual, straightforward conclusion. The declaration that LOST was back in the game would be summed up with one game-changing scene: a Jack flashback that ended with a mysterious meet-up with a woman who would turn out to be...Kate?!
The intrigue didn't stop there, as the flash-forward(!) ends with a future, despondent, alcoholic Jack screaming that they "have to go back." For three seasons we figured the series' end-game would center on the castaways finally getting off the island; the reveal that there was much, much more at stake was simply ingenious, and increased the show's relevance, and significance in the greater TV conversation, tenfold.