REC709 is a monitoring standard for a final image; it is the contrast and color that generally replicates what you see to eye. But, it doesn't necessarily show everything the camera's sensor is capable of capturing. With the latest wave of Digital Cinema cameras, such as the ARRI ALEXA, the Dynamic Range detectable by our shooting devices has expanded dramatically, approaching that of film and pushing past what REC709 is capable of showing. To capture the imaging capabilities of these latest generation sensors, a LOG gamma can be used.
There are many situations where you do not have full control over lighting conditions, such as a bright window in the background or a city street at night. Even with full control, it is still common to color correct a project once it is edited together. If the image is captured in REC709, the ability to adjust the final image is limited by the contrast range already imposed on the image. LOG is a flatter contrast, allowing far greater image manipulation. Take a look at the video to get a sense of the difference.
