The MF-21 is Nikon’s first multi-control back (see this for all the details on Nikon camera backs). All previous interchangeable backs are purely for data imprinting. Designed for the exclusive use by the F-801 (N8008 in North America), this back incorporates a lot of innovative (for its time, of course) functions that facilitate a wide range of photographic tasks. Personally, I found the long exposure feature the most valuable, especially when shooting with very slow films (e.g. ISO 25). Though the F-801 offers shutter speeds down to 30 seconds without the MF-21, it is only in full stops (…, 4, 8, 15, 30 seconds). The ability to control the exposure precisely down the seconds is a great asset for squeezing out every single bit of “data” recorded on the emulsion. Moreover, the long exposure function of MF-21 can be used with the self-timer of the camera and thus removing the need of a shutter release cable (the 2-pin MC-12A) which often sells more than the MF-21 in the used market. Below is a photo taken with my F-801 using the MF-21 to control the exposure. This daytime long exposure was made possible with the use of minimal aperture (f/22) of the 50mm Series-E, a ND8 (-3 stops), a Red-29 (-3 stops), and exposing Ilford PanF 50 at ISO25 (pull -1 development in DD-X). Shutter speed was calculated precisely with the consideration of reciprocity failure.
Another useful function of the MF-21 is automatic bracketing which is pretty convenient for testing the latitude of an unfamiliar film emulsion. Other features include interval timer, exposure data imprinting, and perhaps the most interesting one, focus-priority shutter tripping. The final one would be a great plus for sport photography in the film days and a fun feature to experiment with. Finally, for those who bought a used MF-21 without a manual, this online article would offer great help!
One last note, the Nikon MF-21 uses two 2025 lithium batteries and I strongly recommend users to have a pair of spare ready in their pockets since in my opinion, they run out quite fast.