As mentioned, portraits.
50mm is perfect focal length on your crop format Nikon, and if you shoot wide open at f/1.8 you will get very shallow depth of field which, sometimes looks really good. You can't do that with most lenses.
Focus is obviously critical and with such shallow DoF the slightest movement of camera or subject will throw it out. Focus on the eyes with the centre point; if in doubt, focus on the nearest eye!
The other thing f/1.8 can do is shoot in very low light, but with a high shutter speed and/or low ISO. F/1.8 is over two stops brighter than, say, f/4 - the difference between perhaps 1/8sec and 1/30sec in terms of shutter speed.