I love my SB800 flashes. I have three of them, all purchased used, and I rue the day one of them finally conks out on me because frankly, I don’t think Nikon has improved upon them with the 900 series. Both of these top end flashes do something more than illuminate your subject. You can use them to assist your auto-focus, even when you don’t want to use flash photography.
Even the best lenses will start to hunt back and forth for focus when light levels get too low. Cameras focus based on contrast (which is why it will hunt when pointed at a brightly lit, but blank surface).
SB 800s and 900s have an “AF Only” (located in the AF Menu on the strobe itself) that turns off the flash. It works best when your focus mode is AF-S, or “Auto Focus Single Servo” mode.
So the next time you want to engage in some available, low-light photography but need the auto-focus to work without hunting, give it a try.