The MSCA previous password is used to logon the MSCA on the console inorder to issues TSS commands or shutdown the TSS started task. The MSCA current password is used to signon to terminals and batch jobs.
In very early releases of z/OS, anything entered on the console could be seen including passwords. \
Signing on with the previous password prevents the MSCA current password from being exposed on the console.
Seeing the password on the console is no longer a concern in current times since passwords entered on the console are no longer displayed.
it is a good security practice to use another TSS admin acid to signon to the console and not the MSCA. The MSCA on the console should be used in emergency purposes or as a last resort because all other SCA type TSS admin have been suspended or cant signon to the console for any reason.