Why do we need calibration? Which hardware needs calibration?

https://www.dpreview.com/articles/6497352654/get-more-accurate-color-with-camera-calibration-

http://www.calibratecolor.com/http://www.americanframe.com/monitor-calibration-review.aspx?gclid=CJCGxdzkrMgCFYuRHwodr4YI8Q

http://lifehacker.com/5882632/how-do-i-calibrate-my-computers-monitor-for-the-best-picturehttp://www.xrite.com/colormunki-display

https://m.youtube.com/watch?v=AL-OZUuqr5E