Like many people, I bought my first speedlight (an SB-800) because back then I used slow consumer zooms and wanted less blurry shots when shooting indoors. How naive I was! Yes simply attaching an SB-800, or the SB-900 that I have now upgraded to, will instantly give you easily handholdable shutter speeds in even the dimmest light, but that's only a small part of what a speedlight can do for your photography.
After dabbling with speedlights and Nikon CLS and their wonderfully infinite and rewarding learning curve for a few years, I almost never simply attach my SB-900 and blast my subject with direct light as I imagined it would be back then.
If I want to up my shutter speed, what I'm more inclined to do is to tilt and rotate my speedlight so that the light from it `bounces' off of a ceiling or wall. With no suitable walls or ceilings, you can use a reflector if you have one and a stand or handy assistant to hold it. This technique, called `bounce flash', gives you a much softer, diffused and natural light. Sometimes it can be hard to tell that flash has been used at all and the only slight give away will be that your shots will look so much better than you could have got with domestic indoor lighting.
Bounce flash alone is something that makes a speedlight like the SB-900 worth buying as it opens up flash photography without the risk of the tell tale, pale, washed out look of compact camera flash and surpasses the weak and inflexible onboard flash of even the most expensive SLR cameras. Even if you use fast aperture lenses as I do now, there are still times when a speedlight becomes a necessity. Especially when trying to freeze motion under the weak indoor lighting found in places like the home and sports halls.
The other key area that a speedlight will help with is Fill Flash. This is the technique of balancing your ambient light exposure with the amount of light coming from flash. When done correctly, Fill Flash is almost completely unnoticeable, but what people will realise is that your pictures will `pop' that much more. I won't tell them if you don't!
Yes you can do Fill Flash with on-board flash, but with so little power; your flash range will be very short, often much shorter than you'd traditionally want for portraiture, definitely much shorter than you'd want for wildlife. A powerful Speedlight like the SB-900 allows you to use Fill Flash and maintain the correct distance for professional portraits, even in bright outdoor light.
Ever heard of the golden hour? The time just before sunset where light is soft and diffused. For an hour (or less) shadows soften and contrast lowers to a level that is more easily captured by photography, increasing colour, saturation and detail. You can easily get great results by taking outdoor portraits and wildlife photography in this narrow time window. Photographers in the know, however, know that they don't need to wait on location for the golden hour if the shot that they need won't wait. With fill flash, you can lighten and soften shadows, reducing the overall contrast of the scene allowing you to get the full range of the photo exposed properly.
This means no shadowy panda eyes, no shadows under noses, crow's feet (no not the ones on the actual bird!) and laughter lines disappear under an even illumination and the main subject pops in a way that will make everyone ask how you did it. With a speedlight, you can take the golden hour with you wherever you go.
Even better, with the increased power that you get from using a speedlight over on-board flash, you can use much faster shutter speeds with flash. With AutoFP you can even exceed the Sync Speed of your camera (usually only around 1/250s) right up to its maximum shutter speed (usually 1/4000s or 1/8000s). This has the benefit of allowing you to use wide apertures to achieve narrow depth of field and subject isolation that are not possible when using on-board fill flash in bright daylight. This is the main reason that I carry my speedlight every day. It's not a bodged method of increasing available light as most compact camera users use flash - Fill Flash from a proper speedlight done correctly is an artistic choice - and a VERY important and effective one.
If you're into macro photography then a Nikon speedlight like the SB-900 will help you there too. Either mounted on camera and using the built in bounce card to reduce power or used off camera with the separately available Nikon SC-28 or SC-29, a speedlight's Fill Flash will make your macro shots of flowers and bugs pop in the exact same way. It will also allow you to easily handhold shots at the tiny apertures required for macro to achieve adequate depth of field.
I should also mentioned that the SB-900 also adds 360degree rotation to the flash head as opposed to the oft annoying 270degree rotation of the SB-800 - no more having to remember which way to rotate the head when bouncing off walls behind you! The SB-900 also adds a little extra power over the SB-800 giving extra range, the zoom head now goes to 200mm instead of 85mm again improving range and battery life with long lenses, improved AutoFocus Assist that solves some problems that the SB-800 has with newer cameras such as the D300 and noticeably improved recycling times between shots.
Maybe most importantly, the SB-900 also now has more external controls which make this Nikon's fastest and simplest handling speedlight yet. The instant `on switch' is worth the money alone over the time consuming press... and hold... and wait... of the SB-800.
The most important thing that I have learnt from owning Nikon speedlights, both the SB-800 and now the SB-900 is that flash photography is not a fallback or a compromise for use only when ambient light, aperture and high ISO are insufficient. Flash photography is ambient lights less known brother and he's an important part of your stylistic arsenal.
You must get fully acquainted with the use of flash in its many guises if you want the best out of your photographs and the SB-900, like the SB-800 before it, is a great way for anyone to do this.