I beleive that the current generation of c-41 can live with final rinse. I use e-6 final rinse for all my stuff, including b&w.

There was older generation c-41 films (Not the current portra 160NC, etc) that did need the formalin.

I still have most of a litre I bought about 8 years ago.
When I need it a little goes a long way.
It get used when I home mix my e-6 chemistry, since the formula I use does not have a pre-bleach to get the formalin synthesis going, and in fact the home brew bleach is a c-22 ferricyanide type bleach that will never get any formalin going even if I used a Kodak pre-bleach tank upstream.