I found when I was upgrading an old version that if they were set to zero and I ran setup.asp, it would invoke the section which encoded the passwords for me.
It's only used during the upgrade process from a version of the forum previous to v3.4.xx. It's main purpose is so that if the upgrade process were to fail or timeout part way through, when you restart the upgrade, only those passwords that have not already been processed (M_SHA256 = 0) would be processed.