The one niche that 645 does better for me than a DSLR or LF is the classic near-far scenic with high resolution, and maximum DOF, particular scenes for which I need high shutter speeds (e.g. stopping wave action in ocean scenes, or flowers in a slight breeze), or when I need to grab a shot in changing light or where a tripod isn't feasible (like when snowshoeing or XC skiing).
I'm sure that there are other equally fine cameras out there, but I chose the Pentax 645N and the SMC-A 35mm f/3.5 because they were both incredible price-to-performance bargains. I really like the handling of the camera, metering options, and the big bright beautiful viewfinder, especially once I found and installed an AG-80 grid screen. The images I'm getting on Astia 100F and Ektar 100 are often nothing short of spectacular. The biggest compliment I can think of for any camera is that it doesn't get in the way of the image-making process. The P645N did that more rapidly for me after acquiring it than any other camera I've ever owned in the past 33 years.