Photography is all about light – basically that’s it. Sure there is composition, shutter speed, ISO, aperture, lighting arrangements, soft-boxes etc. etc. But the bottom line is that a photograph is a capture of the light hitting the sensor at a certain time.
A camera assumes that everything is 18% grey. Some scenes average out at 18% grey, but many more don’t. Especially when they feature, snow, deep shadows or studio flash.
A lightmeter I use (Sekonic 358) lets me measure exactly how light is affecting the scene, the balance of flash/constant light and get far better results with less trial and error.