I have a need for it. Currently, I have two 3D printers. I have to connect my Mac in order to not only create what I want to print, but to control the 3D printer. I can connect a SD card to the 3D printer to print from a stored object already created, but I find that problematic if anything goes wrong. The Raspberry Pi would allow me to cheaply dedicate a computer to each printer for all print tasks. If you use 3D printers, you would know they take hours to complete a print job. This would free up my regular computers for other use.
see also Ringing in 2016 with 64 open-spec, hacker friendly SBCs
I personally liked Beaglebone Black, but it's too expensive. Costs nothing compared to servo motors of course.