The extra power of the SB900 class flash like the Yongnuo 568ex really comes in handy when shooting in daylight(one of the most useful capabilities of a real flash) or using modifiers like softboxes or shoot-through diffusers like diffusing umbrellas. There is an advantage to having 622 and 622 Tx transceivers/transmitter over the receiver built in the flash; the pair can be used as a report shutter release also.
I have 3 SB900 Nikon units and those 3 cost about the same as a new D500 so looking back, if I had instead gotten the 3 Yongnuo first, I would have a D500 plus my 3 added 568ex. I used them pretty much interchangeably, mostly in full manual mode but the 568ex is a little heavier than SB900 and the plastic shell is thicker. Both work with iTTL in TTL and TTL BL mode, and adding the YN622 transceivers and/or a YN622 Tx transmitter(about $49 each) and a few low cost modifiers one have a lot of flexibility for studio like shooting with everything but the light stands fitting into your backpack weighing less than 1 studio strobe. There is one advantage of the SB900 series, it can focus the beam width to a field of view of 200mm whereas most flash with beam focusing goes to only 105mm. If you use beam focusing just for spotlighting nearer objects, a folding home made snoot is more effective in controlling the light spill anyway. If you really need telephoto beam width due to distance, getting a remote flash closer to the subject using the wireless controllers is the much better solution than using flash beam focusing. I did a shoot recently for a bride and groom, a week after the wedding, for shots to include in their album that were a bit more artsy with the couple separated in depth by about 100 feet on a dark night with city lights and ocean in the background. It was a snap to get both well illuminate and main exposure for the distant city lights and very dark park in between the couple. A complex 4 flash shot that took 5 minutes to set up, and shoot.
Once you start using augmented light, you will find unending uses for it, indoors or out, daylight or night. Walking around the city during the day, I usually have a flash mounted on my main camera, a D800 and it is particularly useful in mid day bright sun where normally hard shadows and extreme contrast makes non-flash shooting a waste of time. Being able to exposure for the highlights and fill the dark shadows added 6-7 hours of reasonable daylight shooting time when without the flash you would be limited to golden hour in the morning and late afternoon unless in full shade.
Another advantage of the remote RF controllers is high speed sync allowing you to use up to 1/8000 sec shutter speeds instead of normal maximum sync speed of 1/250 sec.
Once you get the hang of using augmented light your creative options increase dramatically. A lot of people, particularly beginners refuse to use flash because they don't like the "flash look". No one does but only beginners take flash illuminated images that look like "flash". Every magazine cover, every beach scene, every commercial photo, indoors or out, however did use augmented light and it looks more natural than shooting with natural light alone.
Getting flash and learning to use it creatively, trumps any other photographic tool for improving images. A cheap low res camera and poor lens with excellent lighting will capture a better image than the best camera and lens with poor or mediocre light. But you read on forums how everyone is chasing high ISO performance for available light. A D3300 with good controlled lighting will out class a D5 or D810 in ambient light any day of the week for image quality, yet little mention of that on the forums in the upgrade mania. Your D7000 with good light is capable of world class images. Mine is, and makes little difference whether I use my D800 or D7000 when I can control the lighting, even when 80% of the light use is natural light. That remaining 20% makes the difference between a snap shot and prized print on a gallery wall.