Why is PSNR used for image quality metrics instead of SNR? I have been trying to determine why is PSNR used instead of SNR for image quality metrics?

I have read through several books and most just say that PSNR is used instead of SNR, but don't give a reason as to why.

I also read these posts:

Signal quality metric

and

Difference between SNR and PSNR

But I still feel like they don't explain clearly why PSNR is used instead of SNR. 

I know that PSNR measures how the peak signal intensity compares to the mean squared error whereas SNR measures signal power compared to noise, but this doesn't help me reach an intuitive conclusion as to why to use PSNR instead of SNR.

Can someone please explain this?
Consider 8-bit quantization. This gives a mean squared error of about 0.577 least significant bit (LSB). Quantizing two images of different average intensities gives two different SNR, because that metric is normalized against the average intensity of the input image, but identical PSNR, because it is normalized against the maximum pixel value of 255. When describing the quality of something like a compression method, it is better to use a metric that is (to an extent) independent of the source material, because it carries meaning even when communicated without providing the input image.