I assume that the reason is to reduce error from having a person read the height from a scale. It would also make the nurse's job easier because they wouldn't need to have to fiddle with the analog scale. I know my kid hates getting his height/weight measured, so anything that speeds up the process is good.

Another potential benefit is that the digitized scale could be directly attached to a wall without regard for it's height relative to the ground. There could be a defined calibration offset stored in the reader, so the contractor wouldn't need to be exact when the bar is installed.