Do you leave your camera on AWB (auto white balance)? Most of us do. I myself am guilty of it at times as well. You ask why not? AWB does ok most of the time, and besides if I’m shooting in RAW I can fix it in post. While this is true with white balance and a host of other things such as exposure, cropping and composition. Sometime AWB will fail you just like most things that are auto-anything. At times you’ll have multiple light sources all with differing kelvin temperatures. These times it’s best to manually set your white balance.
Wouldn’t you rather be out taking photos making art rather than behind a computer correcting things that you could have gotten right in camera? Check out the video below for a couple ways to manually set your white balance on EOS cameras (Nikon will be very similar).