Archival stability, for one.  Conventional colour processing strips the
silver image and replaces it with dyes.  Black and white processing leaves
the silver image intact.

I have no idea how useful the process is - you'd be better to shoot black
and white film than colour if you want black and white images - but it does
work.

Actually, I do know of one usage.  Very old films often process better as
black-and-white images than as colour.  Film Rescue International processes
C-22 and K-11 (-12?) as black and white for this very reason.  The odds of
getting usable images are greatly enhanced.

Jim

One, it may allow use of very inexpensive one-time purchases of film
(cleaning out the photo department of a closed drug store, say) without
paying a lab to process (and without dealing with the vagaries of home
C-41).  Two, it might produce a particular effect someone wants to see
(with my scanner and software, for instance, it's possible to get a
pseudo-color effect from a monochrome image produced with HC-110 or
similar, on color film stock, by telling the scanner it's looking at a
different brand or type of film to induce an incorrect correction for
the base color).  Three, it might be the preferred method of recovering
images on found film, compared to commercial processing and being unsure
what the lab techs will see (one just never knows, these days, what will
be on a roll of found film).  Four, it is the only reliable and
economical way to recover images from very old color film (C-22, K-12,
E-3, etc.), and it's better to practice on something other than the
irreplaceable roll from Grandma's camera.

I looked at it because I can get ISO 400 C-41 film for less than fresh
Tri-X -- but then I found a good deal on bulk Tri-X and haven't needed
to pursue things.  Still worth knowing, just in case -- in these days of
waning photographic options, I can make my own developers and fixers,
but I might be able to buy color film longer than B&W.  Or maybe not...

You can pickup color negative film for cheaper than BW films. Freestyle sells 100ft for 9.99.