Medium Format Cameras

What are reasons to use medium format cameras?

Main reason is the size of the image capturing medium which is either a CCD/CMOS sensor or classic film.
The result is both high resolution with low noise in the image and greater dynamic range with much better transitional tonal values in color.
Both is driven by the larger format compared to 35mm (3-4 times the size) or even crop sensor formats when talking of digital.

Watch the documentary from Karl Taylor explaining the medium format advantages.