Given Clinton and State reluctance to turn over documents, it allows the situation to fester.
As long as Clinton continues to "seem" to act shady, people will dig.
On top of that, the entire email "Don't worry we sorted through them and gave you all the ones we thought were okay" attitude, printing them out just to be a-holes, offsite server in the first place, etc etc....it SO opens her (or anyone doing something similar) to not just digging, but any sort of wild speculation.
Maybe she had some unflattering things said, or sketchy policy discussions at an early stage while they were being worked out, whatever, and didn't want those to be seen. By doing this whole thing, it leaves it open to "SHE HID THE SECRET BENGHAZI EMAIL THAT ALSO CONFIRM BILL IS ONE OF THE LIZARD PEOPLE" and to the right people, there you go. Truth.