As explained in the Wall Street Journal, it's a marker of "economic confidence":
... people don’t tend to quit their jobs in tough labor markets because they’re worried they won’t be able to find a new one. During the downturn, monthly quits plunged to a record low of 1.6 million in September 2009, down from more than three million per month before the recession began. The fact that they’re rising again suggests that workers may finally be seeing signs that the job market is improving.
Quits matter for another reason, too: They’re a component of “churn,” the regular comings and goings that are a critical element of any healthy job market. When people leave jobs in search of higher pay and new opportunities, they open up opportunities for others. When they stop quitting, those opportunities dry up.