What is benefit when convert signed 16 bit to unsigned 8 bit in medical image?
Many image display devices only have an 8-bit DAC.  So if one wants to analyze only what will be displayed on such a device (without added dithering or noise filtering, etc.), one can convert to that 8-bit format and potentially save storage memory.  Many standard image file formats also only support 8 bits of luminance, so the same is true for data planned to be stored or transmitted in such a format.  

However, this process is lossy, and will remove information that a display supporting greater luminance resolution (10-bit DAC, etc.) can display.