You have to understand or have experienced how fragile the early graphics programs were --- basically a .eps file would be a black box of PostScript code, and would have a pixel image preview used on-screen (a classic gag was to use a resource editor to change the pixel preview) --- whether or no it would actually print/image correctly was something one wasn't certain of until holding the actual output in hand, and even then, it only held for a specific PostScript rasterizer/job setting. Sure, it was okay 99.99% of the time, but that 0.01% could be disastrous.
Display PostScript meant that the system used for on-screen display was the same as for actual printed output --- I _never_ had a job on my NeXT Cube fail to print to match the on-screen version. Moreover, one could do cool DPS programming such as custom strokes and fills in Altsys Virtuoso.
These days, PDF and Quartz (née Display PDF) mostly address the reliability (and presumably it's gotten even better since I left the industry), but I miss the programmability. Hopefully, using METAPOST will let me get some of that back.